I've been under the impression that most, if not all, 20-ga. 28" Repros have 3" chambers and that most (but not all) 12-ga. 28" Repros have 3" chambers. And that most, if not all, 12 and 20-ga. 26" Repros have 2 3/4" chambers.
EDIT: Per the linked attached chart the SCC and SSS models were the only 12-ga. 28" guns with standard 3" chambers from the factory. The .410, 26" and 20-ga. 28" guns were standard fare with factory 3" chambers.
http://parkerguns.org/forums/album.p...pictureid=1021