Mounting TV to unusual existing mount

I have problem with using a TV wall mount left by previous owner. Remaining is a wall plate part with a single lug (3 1/8" long, 1/2" hole).

I never saw something partial like that. Usually the hinge part is integral and capped, then wall plate can be bolted into the wall.

Here the wall attachment is solid, but I can’t find anything that has a pin and would secure a pin in the hinge assembly. Specifically, commercially-available part that attaches to TV itself and has a pin to insert into existing lug.

Could someone advise what options/designs are available?