Howdy,
As I understand it, the unthreaded top hole in the bell housing was on the '55-'58 V8 bell housings only. I have a 1963 V8 bellhousing for my '56 3105 and all the holes are threaded. Since all SBC starters that bolt to the bellhousing for 168t flywheels are pretty much the same EXCEPT for the top hole, rebuilders stick on whatever end they have. My starter is supposed to be for a '65 Belaire, but the number on the box covers all SBCs from '55 to '67!
Go figure!
best wishes,
Les
