[Mac VP]

I need to take some pics of both types as the differences are subtle. A rough quick comparison is that the late style flywheels are thinner when you look at the edges. If you measure the thickness of the visible slug of steel, not including the ring gear part, the 49-53 flywheel will be about 5/8" thick. The 1942-48 types will be about 7/8" thick. There are other telltales but this should help you determine which one you have.

Also, in spite of the fact that there are several different pressure plate bolt patterns for the 49-53 vehicles, the flywheels are interchangeable on the 49-53 engines. The big issue is that when you mix the 42-48 flywheels with the 49-53 types, there will always be a misalignment of the starter ring gear. That's when the starters bind up.