[Milsurplus] Crystal Oscillator Unit ID Help

[Brad Hernlem]

I have some crystal oscillator unit items that I am trying to ID. These 
are two rectangular blocks with 7-pin mini plugs on the ends and an 
adjustment access hole in the side (to a ceramic trimmer). The units are 
dimensionally identical but one is marked:

Crystal Oscillator Unit
A-8838300-1  500Kc
Serial 0064:CRV

and the other

RCA DWG #8838300-1
CKM DWG #B-355-11-A
Serial No. 1600A

This second one is also marked 500 Kc. I believe that they are 
essentially identical or were used in the same equipment.

The closest lead that I could find was a reference to an RT-128A/ARC-21 
made by RCA which may have used units like these. Any ideas or info?

Thanks.

Brad
KG6IOE