1917 DWM with 2 S/N's (not years)?

[MT Kid]

Hello everyone. This is my first post, and hopefully I put it in the correct spot. Disclaimer... I know nothing about Lugers! So, here is the story. I was given this 1917 DWM with holster and belt. I field stripped it, and confirmed all matching numbers (except the magazine). The original S/N is 4831, BUT someone overstamped it with a 6633. The overstamp is only in one location, on the left side of the frame behind the take down lever. Have any of you ever experienced this before? My first thought was that it was issued out, returned, re-S/N'd, and then re-issued. I appreciate any information on the topic. (oh, and the usual what do you think the estimated value is, once I put up pictures, etc....) As soon as my wife gets done making dinner, she will shrink down my pics so that I can post them (I are not that puter smart!!!)