I am working on a model 67 20 gauge w/safety behind the trigger. When I look at Gun Parts they show a different butt stock for 12 gauge and 20 gauge. I can find lots of butt stocks for 12 gauge but nothing listed for 20 gauge. Today I noticed at Bob's Gun parts he has stocks that fit 12/16/20. Can any one tell me what the difference is between 12 gauge and 20 gauge? Thanks in advance.