The A1 is what the original M16 and M16A1 had. The A2 is the stock on the M16A2 which came into service in the mid-1980's. The A2 stock is longer, which was slightly good for qualifying scores and bad for practicality. The stocks look almost identical without a ruler. Nearly all stocks on the commercial market today are the A2 size, but Cavalry Manufacturing (fka Cavalry Arms) makes an A1 length stock.