Metal gas tanks before 2012 were made of terne sheet, which is basically solder coated steel sheet. At the steel mill, the steel sheet was hot dipped in terne alloy, 10-20% tin and the remainder lead.

Nowadays terne sheet is steel sheet coated with all tin, no lead. This might look slightly different from the old kind with lead, but not much. The most obvious appearance difference in old and new tanks is new tanks are shiny and old tanks are dull from age, just as old solder or old lead turns dull from atmospheric corrosion. When cars from the muscle era were new, their tanks were shiny, just like new solder is shiny.

If counting off for new tanks just because they are shiny, I would reconsider, because when the old tanks were new, they were shiny also. In other words, what might be considered an "original" finish on an old tank is not original at all - it's a different finish from original because it has aged and turned dull.

Solder is usually roughly half tin, half lead, but repairs to old tanks can be made with solder.

See:
https://en.wikipedia.org/wiki/Terne