Generally the Blomefield guns, introduced late 1780s, have the marks engraved, but the normal guns before that have the cypher, and coats of arms (which this one is missing?) cast on. Can't find any easy examples, but you should be able to check on the British guns captured by those pesky rebels, which I am sure you all know better than me. But if supposed to date from 1762, it should have much better markings, of good quality casting and chasing.
Bob Smith