the .375 is for 9 karat gold. Not sure about the age as the serial number doesn't correspond with the typical W & D (Wilsdorf and Davis)stamping. W & D appears in the late 1900's and I have one from the late 20's with the W & D stamping as well.
If you can post a picture, it would help greatly. Without seeing the condition and shape of the case, as well as the dial and hands, it is hard to give any further insight.