My '60 261 has the same type of cap (extra hole in it). '55 and earlier caps won't work because they use different bearings. I think you need to be looking for a '58 or newer one. My '56 235 didn't have the extra hole, and I believe the caps varied dimensionally from the later ones, but I'm not sure if this would ultimately matter. You will need to get it line bored as well, which may be dificult/expensive as I believe the bearings are not all the same size.
