Silver Pocket Watch Case - Odd Hallmark need help.

Hi there,
I have an old pocket watch with silver casing which has an odd hallmark i havent encountered before.

It is stamped 925 and the letter O but i was unable to identify the makers mark or what assay office its from

Any help or information greatly appreciated

Kind Regards
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This is a London import mark for 1909. The sponsor’s mark, GS, is that of Stockwell & Co, an import agency. GS are the initials of the principal of the company, George Stockwell.