In America the prefix will usually tell you who last serviced or sold the watch, IIRC a practice originally a legal requirement to try and prevent (tax?) fraud but now done on a voluntary basis and organised by the American Watchmakers – Clockmakers Institute.
In UK we are not so lucky and the marks are normally just a ledger or job number and some codes only meaningful to the watch repairer who made them. Just occasionally however you come across something different.
Inside the back of this watch by Review, signed by Jewellers Sharman D Neil Ltd of Belfast and in a Swiss silver case assayed in London in 1926 there are 4 “normal” marks that mean little, except perhaps that one has a prefix NL which could be a contraction of Neill.
But there is also a block of marks all by the same repairer, shown at the top of the page. Easy to read with a loupe but very difficult to photograph, after some serious work with Photoshop we can see that each has a (ledger?) number, followed by “M” and then what must be the month and year and what are probably initials of the workman.
Someone clearly looked after the watch as it was serviced in October 1935, then in 1938, 39, 42, 44, 47, 49 and then in April 1953.