the fineness of 18 carat gold might be shown as 0.75 (since 18 / 24 = 0.75), .750, 750 (without the decimal point), 75% or 750‰.
It is like a percentage sign % but with two zeros below the line indicating that the ratio is per thousand, rather than per hundred used for percentages.
Different expressions of the same number are often seen, e.g.
These are called Poinçons de Maître, which translates literally as "Master's Punches" but are usually called "Collective Responsibility Marks".
These can be used to identify the maker of a precious metal watch case.
These hallmarks were used for plate, vessels and candlesticks etc.
They were not used on watch cases, I don't think there was any Swiss national legal control over the fineness of gold or silver used in watch cases until the Precious Metals Control Act of 1880.
British import hallmarks, like all British hallmarks since 1478, do include a date letter. This means the minimum proportion of precious metal (gold, silver, etc.) in the alloy. The alloy must assay at this standard in order to qualify to be hallmarked.
To ensure that items pass assay, the alloy used by the goldsmith will be slightly finer than the absolute minimum standard.
There is a full description of this system and tables of the marks at Poinçons de Maître: Case Maker's Marks. The Swiss Precious Metals Control Act of 23 December 1880 introduced a uniform system of hallmarking for watch cases to be used throughout Switzerland with the marks shown in the picture here.