51J-4 Mystery Receivers

What was this "X53786" modified version used for? Who made it?
BNC connectors - "To Ext Gain Cont", Null Meter", "Dif Meter"
Filter "X53619-2" with adjustment

j4-20170614_072449.jpg (495920 bytes) j4-20170615_164301.jpg (355470 bytes)

j4-20170614_072304.jpg (285897 bytes)

j4-20170615_164338.jpg (285789 bytes)

This version is probably the forerunner to the LTV G133F (modified 51S-1) for airborne signals intelligence use?

Distinguishing characteristics

Appeared on ebay in 2003 -
51j4-ltv-21.jpg (26678 bytes) 51j4-ltv-22.jpg (28299 bytes) 51j4-ltv-20.jpg (87916 bytes) 51j4-ltv-23.jpg (75613 bytes)
rear panel modified for later ham use?
Appeared on ebay in 2009 - dirty but shares the same characteristics - it has the handles and slides that match up with the G133F
51j4-ltv-01.jpg (25659 bytes)
Tuning knob is clearly a later add-on
51j4-ltv-14.jpg (90740 bytes) 51j4-ltv-11.jpg (109536 bytes) 51j4-ltv-13.jpg (96575 bytes)
51j4-ltv-02.jpg (15706 bytes)
rear panel modified for later ham use