I have found these, along with the Remo/Geha shotguns, interesting as part of the evolution of the M98 and always look at them at shows where they are relatively common. I suspect that they are mostly WWII GI bring-backs. There is no good reference material that I am aware. Many of these are definitely assembled from former military parts in the interwar period and they come in a full spectrum of quality from the truly awful to very high quality. (I recall references that stated that Mauser started using the "Original Mauser" marketing tag line to defend their market share against these "one-offs."
Your example appears to be a very good quality example - engraving, woodwork, finish and overall composition. So, it is difficult for me to decide if this was pre- or post-WWI example but I lean toward the latter. I can't recall seeing any of these that were not 8x57. The non-service caliber and presence/quality of the Krupp barrel seems to either indicate a re-barrel or assembly by a higher end shop.
The attached picture show my two examples, both 8x57:
The top rifle is clearly recognizable as a G98 conversion: Stepped barrel, modified front sight, rear sight base modified from a G98 Lange base, stock sculpted from an earlier G98 stock. Note inlayed wooden medallion over the marking disc position and recoil lug. The military rear sling swivel holes are plugged with dowel. Trigger guard modified for double set triggers and contoured to a more commercial shape. Military lock screws. There are no marks, dates, numbers or proof marks on this rifle except an imperial mark and two digit serial number on the firing pin and a "3" on the back of the safety wind. All of the shop process/inspection marks on the bottom of the receiver have been ground off. The bolt has been bent and the knob ground flat/smooth on the bottom. While the work seems competent, this one is, to me, more of a low end example.
The bottom rifle is an advancement in that the stock is not military and the barrel, while I believe to be military, has been contoured to a commercial profile. Commercial style front sight. Close inspection indicates that the metal parts are military. The receiver retains a barely discernable "Gewehr 98" marking. The trigger guard appears military but more heavily modified. No military lock screws. Not sure if they filled the screw hole, used a commercial trigger guard or used a military guard like a 1909 or some such. The bolt shows evidence of being cut and re-welded in the bent position and knob has been ground flat and checkered on the bottom. There is a 5 digit serial number on the left of the receiver ring. Shop marks are present on the bottom flat. There is an illegible nitro proof mark on the receiver. No other marks. I consider this one to be a serviceable mid-grade example based on workmanship. This is sort of a "plain jane" version of yours.