[rotorwrench]

The 41A blocks had the "59" on the back bell when the post war engines began production due to sharing the same rear shell mold. The front is just like the pre-war 221. As far as other differences in wall thickness and all that, I don't know but they came with 3 1/16" bores when new. They will pass the pencil test since that is a 221 characteristic.