OK, guys, time out here.
If we are talking about a used 870, then you cannot rely on what is stamped on the barrel to determine what type of receiver you have.
About ten years ago, I borrowed an old 870 12 gauge from a friend. The barrel was clearly stamped 2 3/4" or 3" shells. But, this barrel was on an 870 receiver that was made only for 2 3/4 inch shells. The barrel was an aftermarket, that he had bought from Cabelas, to put on his gun so that he could have a Remchoke system. His receiver could not handle or eject a 3 inch shell, even though the barrel said 3 inch.
I would think that the size of the ejection port in the receiver would tell you more than looking at what is stamped on the barrel.
Hope this helps a bit.
Mannyrock