Hi,
Welcome to the Forum.
That would be the mark of Thomas Carpenter. It is one of several that he registered with the London Assay Office, this particular one was entered on the 7th January 1790.
Your watchcase was assayed 1790-91.
For more details of Thomas Carpenter, go to:
http://www.925-1000.com/forum/viewtopic ... 20&p=73597
Trev.