[R-390] AC-3 Coupler

[W4QG]

Hello All,

Excuse me for the slightly off topic post.

I recently acquired a AC-3 HF Antenna Coupler.
I haven't been able to find any information on this device anywhere.
The KWM-2 and 32S1 are referenced in the manual.
The manual is in military style (very similar to a R-390A manual)
but I haven't been able to find a MIL part number for the coupler.
The fellow I acquired it from believed it may have been produced by
Collins, but I have some doubts. Amelco is also mentioned in the manual.
 It is a VERY high quality device, AC powered with a large vacuum 
variable cap.
Since a number of folks on this list have considerable knowledge of this 
type
of hardware, I thought this might be a place to inquire about it.
Anyone have any history on this device? Value? Relative rarity?

Thanks and 73,
Paul W4QG