Need help with speedometer cable end piece I’m lost as heck on this one.

[Cesar Diaz]

1972 Chevy C10 longbed fleetside with 402 and 4 on the floor.

I need some help, finally finished upgrading
G and updating a ton on this truck and got it on the road only for the speedometer to start bouncing around.
Pulled the speedometer wire and it’s not broken replaced it anyways with new wire now nothing. There was a tiny plastic tip on the tranny end of the wire which my local auto part store said I didn’t need and I dropped it somewhere between there and my house. Turns out around do need a new one to this piece. It was worn out. M
Mi removed the speedometer gear at the tranny only to find out that a standard square end wire will not work. Autozone has no clue my local autopart store does not either. The hole the wire goes into is round with a small slot that the wire end piece sits in to make the wire spin. See pics. I have no idea what transmission is in this thing since it’s not the original which was automatic.

Any help would be appreciated. Also how do I find out what tranny it is.
The picture where I’m holding s steel pin goes into the side of the transmission

I filed flat the straight end of a cotter pin and slid it in the gear along with the speedometer wire just as a temp fix and it worked but not sure how long it will last.