I'd look for a little diamond casting mark on the block just above the small timing gear cover. That indicates a post production contractor foundry made block. The replacement completed engines up through at least 1940 were stamped as a normal model A. The incomplete engines (not tested), were not stamped at all. It was up to the purchaser or an authorized Ford shop to number the block. A fair amount of companies used the Ford industrial engine line in there products. Baldwin Gleaner harvesters had a G prefix with there own number. Other companies followed suit on air compressors, power plant motors, pump motors, sawmill motors, etc. I've seen a few of these in my days.