Quoted from MarkG:M-254 is listed in the '66 Bally parts catalog as a clip for a 3/32" diameter shaft.
See https://pinside.com/pinball/forum/topic/what-size-hairpin-clip-used-on-bally-em-counting-units#post-4821794
/Mark
Looks like Home Depot has that size ... https://www.homedepot.com/p/Everbilt-3-32-in-Zinc-Plated-External-Hitch-Pin-2-Piece-808828/204276243