The Stovebolt Site Forums The Shop Area Engine Shop Bell housing

I think it is just a hold over design from the 1937 and earlier cars and trucks that used carbon throw-out bearings. These bearings had an oil cup that needed access to. Once Chev started a design they were slow to change away from it.
On a similar point, bellhousings all the way up to 1962 are drilled and tapped on the front face for engine mounting brackets that were not used after 1951.