A Glock in the DLUxxx serial number range should be a Gen3. I would venture a guess that during the customization process, the previous owner had the finger grooves removed from the front of the frame, as none of the Gen1 or Gen2 Glocks had thumb-rests on the frame. Removal of the grooves is relatively easy to do; the challenge is making the frame look good AFTER the removal.
In checking the Serial Number Project list (another thread in this forum), there is a DLU-series G23 listed, with a born-on date of 1 Jan 2000; the list also indicates that gun is a Gen3:
Hope this is helpful.