If it is a Luxfer tank made in the US in 81 then it is likely the 6351 alloy
Luxfer: Sustained-load Cracking FAQ
Rightly or wrongly some shops will not even fill these tanks.Personally I would not use them. Too much trouble and tanks are cheap.
But if you have a shop that will fill it and it passes Hydro and Viz you could have a deal. At worst you would have a free valve.