Still some confusion on proof markings. The "tons" mark (which is actually service pressure rather than proof) was not used until 1954. Between 1925-54, one change that will help you differentiate from those proofed earlier is that chamber length was marked on the later guns but not on the earlier ones. 1904-25, you would have only the shot charge and a nitro proof. There is also a date code that came into use in the early 20's, but it's quite difficult to read on a lot of guns.

If a 16ga gun is has marks of 1 oz, nitro proof, and 3 tons, it should also have chamber length. But I believe the shot charge was dropped when the "tons" came into use, so that would indicate a reproof.