When the German attack into Russia stagnated in late 1942, some areas of the front returned to a trench-and-sniper sort of warfare that was reminiscent of World War One. The German military actually went so far as to design and issue a periscopic tech rifle mount, the “deckungszielgerät” (DZG). Stereotypically German in design, it was adjustable to fit a variety of different rifles. The original design intent was for the Gewehr 41(M) and 41(W), but the devices were quickly modified to also fit captured SVT-40 rifles, and the K98k could also be used if necessary (although unlike the WW1 trench rifle mounts, these had no mechanism for cycling a bolt action).
It is unclear how many DZGs were actually made, but certainly not many. This example has been heavily restored, but none of the known surviving ones have serial numbers of manufacturer codes (the “DZG” stamped on the side is its designation, not a manufacturer). For more information, see Michael Heidler’s article in Small Arms Review: