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

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j4-20170614_072304.jpg (285897 bytes)

j4-20170615_164338.jpg (285789 bytes)

Another 51J-4 with a similar tunable filter
- thanks to Tom W3TA

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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 -
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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
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51j4-ltv-02.jpg (15706 bytes)
rear panel modified for later ham use