Ed, I never remember seeing a serial number on an 870 barrel, but do remember seeing them on the receiver, left side. Just to be sure I checked the two I have in my safe. The first is an 870 TB vintage 1978 - serial number on the receiver, left side, none on the original barrel, just some proof marks. The second is an 870 Express, vintage 2005 or so, same story. I also have a slug barrel vintage 1980 or so, no serial number, and a barrel of unknown vintage that is marked "Full" and 3", but was damaged and cut off with a hacksaw. Same story, proof marks, some patent numbers, but no serial number.