It’s a London import mark. For some reason your picture of the full mark did not appear but is available through the link. This shows a date letter m for 1927/28 and the very worn importer’s mark, GS, of George Stockwell of Stockwell & Co.
I tried to add two images but only one appeared. She missing one is: This is the first time I have used this site. We are presenting our grandson with his great grandfather’s pocket watch on his 18th and wanted to establish as much information as we could. Thank you for your quick response:
