My serial number is 524688 for the 6" and 571036 for the 4". I do not have a letter, this is circa 1926 or 1927 from what i am told. Sorry for the delay I just checked back in. They are a couple of my favorites because they look brand new. These were beater guns not collectibles like I have said before. True survivors.If you would be so kind, what is the serial number (ballpark # is fine) on your 1926 M&P? For sentiment reasons I'm in the market for a revolver from 1926. From what I've gathered so far the serial number range for 1926 is approximately 510000 - 560000.