Mark seen regularly in serpents with just a silvery appearance (4)

I haven't cracked this one yet — but the brilliant team at Fifteensquared have:

Alternate letters of sErPeNtS I’m not a fan of these non-acronyms used as solutions – but this is  rather clever: EPNS stands for electroplated nickel silver, which would have a hallmark – maybe a serpent is one of them but I didn’t have time to trawl through this list