I think you are missing a piece. Looks like a thick, loose fitting metal "clapper" hangs there to ding the bell. Maybe it fell off and rolled under the cab floor - worth a check. Looks like you could make one similar from some round stock (complete with what looks like a chamfered hole). Or perhaps you could just find some fender washers of appropriate size and glue them in a stack to make it work?
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i wouldn't bother taking up the floor. I meant looking under it with a mirror and flashlight or fishing under it with a magnetic wand. If it's under there, it would probably dislodge itself if you put the cabinet on end, too. Also, don't think you need to disassemble the end of that actuating arm. I think whatever hung on it, just had a hole diameter slightly larger than the "button" head on that arm (that's probably why it fell off in the first place maybe during transport). You could just secure it on there with an E-clip or Hair Pin Clip.

Your Williams # A-5505 Clapper Arm is indeed missing the "loose fitting" roller washer that actually strikes the bell. These were originally riveted on the clapper arm. Perhaps a couple of nylon washers could be pressed over the head of the rivet to work?

Your game has A-5505 clapper arm attached to B-5492-3 drive arm. Those parts were discontinued around 1966. They were replaced by 1A-3610 (or 01-3610) clapper arm attached to A-6488 drive arm. Both the old unit like yours and newer units use the same bell: 1A-1179A (or 01-1179A).

Consider it a good day. Usually you find a dozen cigarette butts and the skeletal remains of a mouse. Hang a metal washer on that arm and claim victory.
Here's a picture of where somebody did just that on their machine. It's a Heat Wave just like yours, too. Found it on IPDB!

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If you look closely, you can see that he affixed a cotter pin to keep it in place.

That's an interesting way to do it. The important thing is how it sounds. Post a short video clip of it in action.