The date letter could be "T" for 1774. The visible parts, especially the "tail" would fit in the character. Not sure about the maker's mark. Is it W+something (for example WF for William Fearn) or upside down BM?
I think WB or WH look more likely. No standout candidates for these initials but William Barrett I, smallworker, entered 9/3/1771 and with a long career through to at least 1793, is a good possibility.