In response to the earlier question of the hole in the Sherman turret I posted a question on the. G104 - the Sherman Register mailing list
These guys are a fontain of knowlwdge on all things Sherman, They replied with the information below, whih has been cut and pasted with their Permission. Many thanks to paul Roberts and Gary binder for taking the time to reply.
The original question was posted here.
The 2" bomb thrower was, indeed, initially a British thing, but, once it was incorporated for the British contracts, it was adopted directly for US production. Once in production the hole and the mortar (because this was really the 2" mortar with a breech loading mechanism) could be found on all subsequent turrets. Like the turret pistol port, I do believe it went in and out of production. I'd have to closely reread my Hunnicutt to be sure, but this is what I remember.
The 2" smoke mortar was adopted for US use in 1943 (if I recall correctly). It appeared on the 75mm, 105mm and 76mm turrets. Not long after VE Day there was an Army Technical Bulletin that ordered the removal of the 2" smoke mortar, removal of the ammo box (inside the loader's side of the turret) and the closure of the aperture by welding. All the 76mm turrets on display vehicles in Nebraska have had this done. The quality of the closure varied according to the people doing the work. Some are filled and ground so that you can barely tell the aperture was ever there, some are almost crude. That's why you don't see the smoke mortar aperture on tanks that stayed in service after 1945. It's not that it wasn't there, but it was filled in.