Your letters might be X Q B. there is a 1914 ser.no 468 and a sn 1426 reported with those letters.
Looks like a decent piece. May I suggest that your thoroughly clean inside the breech, inside the firing pin, AND under the extractor. Places some collectors miss.
http://www.wonderhowto.com/search/luger/
Also if you're going to continue to shoot it, get a new magazine. Those wooden bases are getting fragile.
FN