Fly is correct
13.5 would be marked at the time of proof and is the bore, not the choke = 31 gauge. 32 gauge is .526"
The breech end of the chamber for 32g is .574" tapering to .562"
28g is .626" tapering to .614"
http://www.dave-cushman.net/shot/shotshellloads.html
What does the chamber measure?