This is an Omega trench watch, reference number 568.13. The case serial number is 538-0478. The movement serial number is 133-6661. There appears to be an “H” in front of the movement serial number. I am unsure if the case material is silver or steel. It has an attached fob with blue enamel.
3.5cm x 3.5cm x 1.5cm
Omega - WW1 Nickel Trench’ Wristwatch,, Swiss circa 1915
the 35mm nickel case, with 24hr white enamel dial with luminous Arabic numerals and inner red Arabic numerals, with subsidiary seconds, red twenty four hour inner track and fine blued steel hands, manual wind possibly Calibre 12 SB movement, on later black leather strap.