It's possible, but it has a full stop after it, bioth on the clock and in the book.
However looking in the Yorkshire book it does seem to be Yarm in Yorkshire. There were a lot of Thomases, the first born about 1744, the last in Yarm listed at the shop in Front street in 1840. This could just about be by any of the three in Yarm, but I think by the first.
That's true, but people seem to have been rather free with full stops on dials back in the day, not always meaning an abbreviation. I have one marked King's Lynn with a stop. The dial of the OP's clock has plenty of room to write Yarmouth in full, if that had been what the town was called.