Interesting.
The pole fitting looks like one of those male ends that were designed to fit into a "Bell" shaped socket, the socket having a pin that engaged the circumferential groove in the pole fitting.
If so, that's the really old design, not all that often seen today.
More modern designs use a pin on the
mast car that plugs into a female socket on the pole.
Still others, (that you obviously don't have,) simply use a "hook" on the pole and a "ring" on the mast car.
If yours is the "Bell" style you might have a hard time finding that piece of
hardware.