I just bought my second "double hump" model 520 with the suicide safety in 12 gauge (so I can have a matching set). I enjoyed restoring (almost done) my first one but I am having problems finding certain parts for my second. I watch the auction sites and Numrich for parts in 12 gauge but many are getting hen's teeth rare.

I understand some parts are interchangeable between the 12 and 16 gauges (Isn't the frame the same?). I know the barrel is a different size and perhaps the bolt but does anyone else know for sure? I would like to get a better stock and operating handle bar and some other parts but not sure if the magazine tube diameter is the same and if the operating handle bar or action bar is the same length. If many parts are interchangeable then I can expand my searches. Can someone enlighten me?

I read that the 20 gauge versions are vastly different (smaller) so I am not going there. Just 12 and 16 gauge interchangeability.

Thanks for any help in advance.