as Rory says... the number/ letter combo is original... to check if your stock is correct to your gun... the number/ letters should match on the butt plate, stock , and when the stock is removed the same combo will be stamped on the left side of the lower tang. ALso, depending on the serial range... my experience is that the forearm should be stamped with it as well (or often something very similar +/- one letter or number, etc) up through the around 650,000 or so but will have to check my stats again to verify that.
After that the forearms and stocks are often completely different number/letter sequences but the forearms will often have the last digits of the serial written in chalk on them.
Here is where the stamping is located on the tang
stock with partial serial and stamping sequence also in chalk
and butt plate
didn't see any pics in my photobucket account of a forearm with the chalk...
Hope that was not confusing.