This version of the R-388/URR was manufactured by Collins for a commercial or Ham application. The cabinet is rare. Most Military units were rack mounted. The 51J-4 was designed for
500KHZ. IF (intermediate frequency) operations therefore the filters designed for the 75A3 will not work in the 51J-4. The 51J-4 was designed before the popularity of SSB (single sideband) and therefore does not
include a product detector. Without two much trouble you can receive SSB by turning the BFO (beat frequency oscillator) on and slowly tune for intelligibility. Another picture below shows a ISB-1 by Delta Electronics that has been aligned to work with the 500KHZ IZ . The ISB-1 is a product detector and audio amplifier. The ISB-1 is shown mounted in a cabinet with a small speaker panel. In reality I think the 54J-4 does just fine receiving SSB by using BFO injection.