Most shipped with a 30" full choke. What's this barrel he's trying to sell you?
Also, it doesn't REALLY matter if there is a stamp on the barrel. Contact Remington if you're concerned, but I wouldn't care. You can always measure the chamber to confirm length. You shouldn't need a stamp to tell you 'don't use a 20ga shell!'
Any 3" receiver will say 'magnum' engraved on it, and I believe the last digit of the serial will be an 'M'.
as a former Remington warranty station employee-x2
He said that's what the lack of serial numbers meant. That threw up bells for me, hence I thought I'd asked people here