The Stovebolt Site Forums The Shop Area Engine Shop curiousity about engine numbers stamped inside the block

I was looking inside the block and in the side wall I noticed some numbers stamped into Looking from the crank side up as the engine is on an engine stand. Between the front cap and the 2nd main cap from the front its stamped 56 A 81, from the 2nd main bearing cap and the thrust main bearing cap its stamped 56 B 71 and between the thrust main bearing cap and the rear main cap its stamped 56 C 56. Just curious what it meant.