SP-600 DESIGNATION'S PRODUCTION       SUFFIX #            NOTES DATE Feb.   1953                 SP-600-JX-26          Std. frequency range, 540Khz -                                                                              54Mhz. Signal Corps.                                                                                             R724C/FRR, order no.                                                                                           3376-Phila-52. Effective upon                                                                                Signal Corps. approval of TAR#10                                                                        dated 2-12-53. Replaces JX-14. March 13,  1953         SP-600-JLX-27        Std. frequency range, 200 -                                                                                    400Khz. 540Khz - 29.7Mhz. Oct. 5,  1953               SP-600-JX-28          Std. frequency range, 540Khz -                                                                              54Mhz. Signal Corps. R-620/FRR,                                                                        order no. 25693-Phila-53-61,                                                                                 contract DA-36-039-SC49453. March 12,  1954          SP-600-JX-29          Std. frequency range, 540Khz -                                                                              54Mhz. Made for C.I.A. contract                                                                            no. XG-1178. Dec. 28,  1954             SP-600-JX-30          Std. frequency range, 540Khz -                                                                             54Mhz. Diversity receiver.                                                                                      Replaces JX-17. Dec.   1954                  SP-600-VLF-31       Std. frequency range, 10 - 540Khz.                                                                        Very low frequency receiver. No                                                                            Xtal frequency control. Dec.   1954                  SP-600-JX-32          Std. frequency range, 540Khz -                                                                              54Mhz. Black wrinkle finish front                                                                            panel with white engraves lettering.                                                                       Made for Mackay Radio, their                                                                                order no. M-41666, Hammarlund                                                                           production order no. 2467-300.                                                                              Internally the same as the JX-21. Dec.   1954                  SP-600-JLX-33        Std frequency range 100 -                                                                                      400Khz, 1.35 - 29.7Mhz. Aug.  7,  1956               SP-600-JL-34          Std. frequency range, 100 -                                                                                    209Khz, 540Khz - 14.8Mhz. Made                                                                         for C.I.A.. Their order number.                                                                               XG-1765. Aug  1956                     SP-600-JX-35          Std. frequency range, 540Khz -                                                                              54Mhz. BFO range 0 - 10Khz.                                                                                U.S. Navy R-274B/FRR, order no.                                                                       NObsr-71369. Oct. 3,  1957                 SP-600-JX-36          Std. frequency range, 540Khz -                                                                              54Mhz. Same as JX-21, except                                                                             for addition of audio input jack on                                                                          rear of chassis. Made for F.B.I.,                                                                            their order no. FBI-16876, their                                                                              contract no. J-FBI-3873. Oct.  9, 1957                  SP-600-JL-24          Std. frequency range, 100 -                                                        (SPECIAL)          400Khz  1.35 - 29.7Mhz. No Xtal                                                                           frequency  control. Supplied to                                                                               NAVALSHIPS 91661,                                                                                            R-274B/FRR PER PL-33910-1. Mar. 28,  1961               SP-600-JX-37          Std. frequency range, 540Khz -                                                                              54Mhz. 25 cycle power supply.                                                                              Otherwise, same as JX-21. Mar.    1961                   SP-600-VLF-38        Std. frequency range, 10 -                                                                                      540Khz. Very low frequency                                                                                   receiver. Same as VLF-31 except                                                                         for 25 cycle power supply. July  21,  1961               SP-600-JX-39          Std. frequemcy range, 540Khz -                                                                             54Mhz. Made for F.A.A. contract                                                                           no. FA-2338. June 1969-1972            SP-600-JX-21A        This was the last series of                                                                                       SP600's manufactured. It had the                                                                         std. frequency range, 540Khz -                                                                              54Mhz. Has 22 tubes vs 20 in the                                                                         other models.Also, a seperate                                                                               product detector, LSB, USB,                                                                                 CW, MOD switch.Appearance                                                                               was different from other SP600's                                                                           in that the knobs had no metal                                                                                skirts, the front panel engraved                                                                              with markings for Xtal phasing,                                                                               selectivity, bfo, audio gain, rf                                                                                 gain. also is marked JX-21A on                                                                             the front panel.   The dates came off engineering revision "sign off" sheets. Robert Fowle obtained from the late Wayne Cordell, last Administrative Sales Mgr for Hammarlund, so are only approximate dates. Les Locklear condensed & retyped it all for this format. During the Vietnam War, several "Hybird" SP-600's were produced with 9 tubes, printed circuit boards and 200Khz bandpass filters. They were connected to video recorders, usually in banks of 5 receivers to seek signals in the HF spectrum. Tapes were then sent to Fort Meade, Maryland for interpetation. Les believe's the military designation was R-1151/GR. This is similar to the MansonLabs R-1230(R-390A's). Hopefully, this will help SP-600 owners in identifying the various model types of the Hammarlund SP-600's.