The letter that is in front of the US Patent # is the yr of manufacture. It should look something like Z-US Patent 27384. I think that information is in the FAQ's. Some Toppers did come with centerfire bbls .357Max ,22Hornet 22 Rem Jet and 30-30 Win. The factory will not fit bbl to the Toppers though. If it originaly came as a 20ga only it may have the big fat firing pin and that could be problematic. Kurt