Winchester is the only company I can remember seeing marked as the maker's product instead of store brand without a serial number-I had a model 270 pump .22 with no number.
It makes it really hard to get things like beads and chokes because I have to first determine weather I need the winchester part or browning part.Yep I've got a pre-1968, 20 gauge Mossberg 500 without a serial number too.If the "polychoke" is original, it should say C-Lect-Choke on the side of the barrel, where the rest of the info is.,.I think current barrels will still fit, ask a gunshop if they have a used, current style 20 gauge 500 you can try to swap barrels with.Will have to look at my brother in law's Coast To Coast version and see if it's numbered, I don't remember for sure.
I've had several un-numbered guns myself, only one now(old single shot 12 gauge inherited from my Grandpa).
), both of which sold many brand labeled guns made by companies like Mossberg.
The Mossberg 800 came in different calibers which were designated by a letter following the model number.
Nice option but it must not have been all that popular, all the C-Lect choke Mossbergs I see are 40 years old.
If you haven't bought it yet just check things that you would check on a more recent gun, but look a little closer(for things like rust or obvious excessive wear).
You'll have better luck with this request if you make it clear that you plan to buy another barrel from that shop and want to make sure it will fit.