You have a Karabiner 98k. The WWII era Mausers were marked with letter or number codes to hide where they were made, while WWI era mausers had the manufacturers names stamped on the receivers.
Marking would be DOU which means it was made at the BRNO mauser factory in Czecholoslavakia- the ones made at BRNO and Oberndorf were top of the heap for quality. BTW, BRNO still makes top notch mauser rifles (CZ).
A complete unmolested (or even un-rearsenalled- the Russians and Yugoslavians rebuilt boatloads of these, and CZ-BRNO built a lot of them from leftover parts for export) rifle would be worth upwards of $500 depending on condition. Drilled and tapped for a sporter scope, they aren't worth all that much unfortunately.
edit:
It was modified - an old scope was mounted
What kind of scope is it? If by rare chance that its not a sporter scope, you might have struck gold.