A RF Modulator of unknown origin and purpose...

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Hi folks,
a few weeks ago I stumbled upon this rather mysterious little Piece of hardware. It seems to be a RF modulator from the looks.
The PCB has a P/N: 55.301.D1
The backplate / connector, I'll get to that in a second, has P/N: 55.303.P1

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There's only one cable to connect, probably the video signal? It seems Vert./Hor. is purely adjusted by the onboard adjustable resistors, just a guess.
Now, I bought it unknowingly with a dozen or so large boxes of random hardware and a few computers.
My first thought was that maybe it belonged to a Commodore PET, as the accompanying PET has a cable soldered to its video feed, but not to the Hor./Vert. lines.

I was about to try out this theory when I, once again, took a closer look at the little machine. This time I noticed symmetrical scratch marks on the protruding part of the backplate, as if it were put into a socket of some kind. Note the single copper trace leading to that socket, possibly used for power supply. The thing is, this socket doesn't fit any port on the PET - Nor any other socket that comes to mind :?:

If anyone wants to guess as to its purpose and origin, shoot away! I'm at a loss as what to do with it right now ;)