The R-270 is a BC-794 modified for dual diversity operation with an external master oscillator. Multiple R-270s, the master oscillator, and other equipment were part of the AN/FRR-12 diversity reception system. The old JANAP 161 says that the R-270's in the system are crystal controlled, probably with the MC-531 kit. The AN/FRR-12 system consisted of one six foot rack, two R-270s, and 2 RA-74-D power supplies.
Improvement Kit MC-531
The MC-531 was a add-on kit to add crystal controlled reception to the SP-200 series receivers. Three switch selected crystals, and a crystal trimmer, were provided.
Lattice crystal filter replacement for Lamb single crystal filter
July 1960 QST, "Improved Selectivity for Older Receivers, A Low-Cost Half-Lattice Crystal Filter" by John M. Palmer, W1SGN. Uses surplus "numbered" crystals, fits in the original Lamb filter enclosure. Crystals 49, 50, 51, 52, 53 and 54, with labeled freqencies of 24.9 to 25.4 Mc, are used for the Super-Pro (465 KHz IF); crystals 41, 42, 43, 44, 45, 46 for the HQ-129X (450 KHz IF). Net result is a filter suitable for narrow AM or SSB at the expense of the narrowest CW bandwidth of the Lamb filter. Thanks to Alvin Bernard, NI4Q, for a copy of the article. My SP-210LX has this filter.
Jon Oldenburg, email@example.com, reports a SuperPro (no nameplate, SN #5243 stamped on chassis) with coverage from 0.1-0.2, 0.2-0.4, 1.25-2.5, and 2.5-5.0 MHz. This is unlike the SP-210LX/BC-779A which share the same two vlf/lf bands, but then cover 2.5-20 MHz in three additional bands. If you have any additional info on this model, please contact Jon.
Rich Post was restoring a R-129/U SP-200 variant. It covers 300 kHz to 10 MHz in 5 bands.
Howard Radio BC-779-A
Jeff James, firstname.lastname@example.org, has a BC-779-A produced by Howard Radio. The radio, which served on USS Grafton, came complete with all original documentation in a ship's binder.
John Oldenburg, email@example.com, also has a Howard-produced BC-779A, serial 7469, contract #21109-phila-42.
Australian-produced BC-779 receivers in WWII
AMR-200 by Eclipse Radio, Melbourne AU. (David Prince, firstname.lastname@example.org)
Post-war coversion to miniature tubes and internal power supplies
(Bill Sorsby and R.F. Buchanan). Several conversion/updating articles were in CQ Magazine:
The Ultimate Conversion of the Super Pro Receiver; Charles E. Reed, W6FHB; CQ, April 1961, pgs 38-46
Save Your Super-Pro for SSB; CMDR Paul Lee; CQ, September 1958, pgs 52-55 & 104-105
Souping the Super-Pro; L.E. Geisler; CQ, December 1957, p. 30
Unknown mods performed by American Television Labs, Chicago, stated on front panel metal tag. (Charles Buttacavoli, LBFJ60A@prodigy.com)
Additional information may be found at Alan Klase's Super-Pro Data and Richard McClung's Cross Reference for Older Hammarlund Super Pro Receivers pages.