From all the original ones I've had, it appears that it started as a field-mod on the first batch of M2's in service that involved a simple, but often jagged, torch-cut. The field mod later evolved to a torch-cut with a piece of split pipe welded over the edge. It then followed with a 'factory' mod that was a smooth, wider, factory-cut in the fender well plate. Then later, a piece of cast steel was welded over the factory-cut edge. It looked kind'a like a huge Twinkie(?). I'm not positive, but the cast block may have been added after WWII during the 'rebuilds'.....