Using Jeff Noll's fine book on Imperial German Regimental Markings, it would translate as follows: Feldsignal-Abteilung, Kompagnie 4, Waffen 19 or Field Signal Detachment, Company 4, Weapon number 19
However, those are WW1 Imperial German Army unit markings and are out of place on a 1939 Mauser. It would seem that you have a Luger with a WW1 era frame and later upper; at least the toggle train. Clear, detailed pictures of both sides and top of your Luger would help to determine exactly what you have.
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