Could this be the rather unfortunately named Jasper Cunst ?
He entered an incuse mark as a gold worker on 21 August 1776 (Grimwade p309) and on page 482 Grimwade states
"his main production may have been gold watchcases"
If the watchcase is gold, then Cunst is the prime suspect.
Even if if it is silver, he is a strong possibility.
For convenience I've clipped and embedded the marks from the prime posters image .