I didn't think it was cylinder bore, since as zippy said, others I've had said "CYL".
Perhaps your gun had an adjustable choke (thus unmarked) and it's had the barrel shortened.
That's exactly how I made an extra "skeet" barrel for my 1100 Trap: Cut at the rib post and threaded for screw-ins; but, it's still marked "Full."Just a guess here but I'm thinking they make all of them to 30"-32" to start then cut them down to whatever final length they want before threading for choke tubes. In order to use the same vent rib on all models they have cut the barrel so that the closest vent rib post matches the end of the barrel.