I have a cab in pretty sure I've dated as a 78 based on the speakers. It has a regular towel bar on the back and a rectangle 1/4 cable connector plate on the back. I'm no expert by far but your cab looks quite a bit different than mine from the back.
Any reason to believe it's not Ampeg? Maybe older or newer?
Edit: I've seen other three-screw mounting jacks doing some searching.
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