[Collins] 75A3 accessory?

[Guy Olinger, K2AV]

Dear Collins Guru's

The 75A3 I recently received came with a little mystery.

It was plugged into the NBFM adapter socket. However, when reading the
manual I see that the xtal calibrator is a single 6BA6, and the NBFM
adapter is TWO tubes, a 6AU6 and a 6AL5.

The adapter I have has a single 12AU6A on top of an IF can that
presumably holds circuitry. When it is plugged in, it is in a diamond
aspect with the back of the chassis, rather than parallel.

When examined, the socket has been rewired.

Might the beastie be a product detector?

If so was that a Collins mod, and is there a schematic around?

Many thanks & 73,

Guy.