Hi BobJ . . .
It looks to me like there is only one part number for that cap used on all full pressure 53-55 235s and 261s. The parts wiki:
'57 Master Parts Catalog shows the casting number 3836538. Is that the casting number you have in hand (but broken).
The good news you should be able to source a part from a (more common) 235 that is perhaps unusable and headed to scrap. In mid '55 they changed the bearings from being staked to using a tang to hold them in place.
I suspect one could modify a tang type bearing cap by drilling the relief on a drill press. A paint dab (on the raised stake-button) on new bearing shell could be used to locate the spot.
I am aware of one 261 that was heavily modified by a Club member. He replaced both the intermediates with heavier billet bits, custom-cut and then align bored.
regards,
stock49