5th Earl is probably a good candidate. Here's a picture of his arms on a book binding:
https://armorial.library.utoronto.ca/st ... ers/STA011He wasn't made a knight of the Garter until 1805 so if we assume that the engraving is from 18th century it shouldn't have the Garter and it doesn't. But the initials don't look that much 18th century to me.
Different members or branches of the same family used slightly different arms. For example Stanhope, Earls of Harrington use a crescent in the middle to differentiate their arms from other Stanhopes and younger sons would use marks of cadency. A full heraldic achievement would include supporters, motto etc. and of course there are some artistic variations too so arms are never executed in the same manner.