I'm looking for a used back for a 501C and I'm seeing a few different model numbers (30074, 30147, 30212, 30213) which I'm guessing give some indication as to it's vintage, 30213 likely being the latest. Is there site somewhere that will help me identify what years the different models were manufactured? I realize there is the date code on these, but alot of sellers don't state what it is or show it in an image.