Shotgun World had an Ithaca 37 vs BPS 28 gauge thread a while back. all said the 37 was a '28" frame while the BPS has a larger frame that is used for the 20, 28, and .410.
I just bought a BPS 28 last week for roughly $500.00. 28" VR barrel w/3 chokes and the manual. It was marked "used", but I have no idea if it was ever fired outside of the factory. It feels really nice, I can't wait to give it a go the next time I get to shoot.