I have an extremely small sample, but see the 410s with an A serial number prefix. I see the S prefix on 16 and 20 gauge guns.
I think you may be onto something here. In the pictures I've saved of Lefever Nitro Specials the .410-bore forearm irons have the A, the 16- and 20-gauge forearm irons have an S and the 12-gauges don't have a letter. The same seems to be generally true for NID extractor forearm irons -- A on .410-bores, S on 20- & 16-gauge, none on 12- & 10-gauge, and 28 on 28-gauges. Pretty hit or miss on ejector forearm irons.
