I think (me) that the A indicates an assembly, ie a fitted block with pistons but no crank, cam, etc. A B suffix indicates just a block, no internals, C indicates a complete shortblock, crank, pistons, cam, etc. Its the digits that indicate when the block was made, but that depends on how many actual CE engines (or parts of engines) were made in any particular year which is very hard to nail down. This method would result in a maximum possible number of engines from any one engine plant of 30,000 per year.
Other thoughts are that since each plant built roughly 30,000 blocks each based on the sequential numbers that each plant was asigned. The first run thru of 30000 engines (or parts thereof) did not have a letter prefix, the second 30K had an A prefix, the third a B, etc. I've seen letters as high as C on CE stamps, so that means that Flint V8 plant (or tonowanda, or the Flint 6 cylinder plant) could have made 90,000 to 120,000 CE components each year. Thats a bunch of engines and/or blocks being replaced each year.