Featured Abraxas (?) Gnostic Amulet (?)

Pewter is primarily tin. According to this:

https://www.xometry.com/resources/materials/what-is-pewter/

Originally, pewter was a 70% tin and 30% lead mix. However, the lead reacted with acidic food which it came into contact with, which earned it the name “black metal.” This is because it became darker with time. Now, pewter is generally 91% tin,...

I suspect the metal of your amulet is mainly lead, probably not pure, possibly alloyed with tin.

I'm persuaded your piece is Roman era, most likely Roman Britain. There seems to be so little interest in these amulets from scholars of gnostic artifacts, it is really improbable that anyone forged copies.