The Swiss permitted a nine carat marking on Swiss made cases bound for the British market. Both the case back and case head of your Rolex carry the mark....I am more puzzled by the ten carat designation on the case back, but I am no expert.
The double opposing "FF" hallmark on your case back is that of the Glasgow assay office. The letter "B" is, I believe, the Glasgow date letter for 1950. However, the Glasgow assay office did not change its date letters on the calendar year. Rather, the Glasgow assay office date letter year runs July 1 of one year to June 30 of the next. As such your watch could be either a 1950 or 1951. Here is an image of the Glasgow assay office date letter for July 1, 1950 to June 30, 1951.
Click to expand...