A Pocket Watch for use on Nuclear Submarines.

I was a little sceptical about this but the story checks out. The watch is a standard size 16 Waltham 1908-1609 with 9 jewels, a Swiss lever escapement and a Breguet sprung cut compensated balance with double roller. It is adjusted for temperature and in three positions. The serial number dates it to 1942.

As confirmed by the catalogue number it was originally supplied to the Royal Navy during WWII with luminous hands and face, it was retained at the end of the war, unlike around two million pounds worth of surplus watches and clocks that were sold off.

Later, following the introduction of Nuclear submarines in 1960, it was given non-luminous hands and dial so that the relatively high radiation levels (by todays standards) would not confuse on-board radiation detection equipment. The case was then stamped "Non-Lum".
The American screw backed case is in base metal case and was made by the Star Watch Case Company, it is stamped with the military catalogue number, the Military property mark, the service allocated serial number and "Non-Lum" as described above.