The only thing now preventing the two frames from being separated is this gear.

It is fixed to the shaft with a taper pin.

A taper pin is a pin that tapers from a wide end to a narrow end used to affix parts onto a shaft. The machinist would put the gear on the shaft, drill a hole, then taper the hole by using a tapering reamer. Then the machinist would put the taper pin in, give it a few gentle whacks with a hammer, and grind off any end sticking out too much.