Imo, your rifle is probably authentic and quite rare, - "true conversions" like yours are actually rare. 95%+ that people call G98/98k conversions are nothing of the sort, most are simply recycled G98 receivers, even the SS conversions typically use an ordnance spare 98k barrel (though they are generally still conversions as the SS retained elements of the original G98 in many cases)
Very few, relatively, go this extra mile and shorten the barrel, fewer still shorten an original stock to 98k. Though your rifle apparently also followed this easier method of using ordnance spare components were practical. I suspect these rifles had their original barrels shortened because an ordnance spare wasn't handy and the rifle and barrel was serviceable, which suggests it was done by a ordnance staff or smaller HZa (a large Wehrkreis depot would probably not have this problem, - or maybe they had the time to do the extra work if it was early enough. We know the ordnance depots were combed for men later in the war, if it was done in 1940 maybe they had the time, though the NS era isn't known for their frugality... Imperial era they re-used everything to the point of absurdity, but the superman generation tended to resort to the parts bin whenever possible)
Anyway, of the other known rifles like yours they have some similarities, including in serialing fonts (larger, more generic, similar style); one rifle in particular stands out in some of its similarity, Danzig/1916 4597 nn, 1920 property marked, but is the only mostly matching original in good condition known. It carries a HZa depot marking at the wrist, not decipherable unfortunately, looks like E(eagle)/HZa*an1 or maybe 4, hard to say, the owner tried to go the extra mile on photographs, but still in was too indistinct. Has a lot of similarities, a Walther e/359 ordnance spare stock (like yours seems to be), the bolt a recycled Amberg bolt, bent and re-serialed to the rifle. It was posted somewhere in 2015, probably here or Gunboards-Mauser as I do not go elsewhere generally. There was a discussion then as I recall, which like always discussing these had a lot of skepticism, but it is clearly good and so too is this one probably because of the similarities. I would still like to see a fireproof once in awhile for these...
I would look with a magnifying glass at the stocks wrist and lower buttstock for a acceptance, something like I mentioned before, E/HZa***, if one is found and it is authentic it would greatly add to the value as it would support its authenticity. When dealing in reworks the less doubt the better, all will have some doubt, - being done by dozens of facilities by hundreds of individuals (armorers) over a wide variety of circumstances (peacetime, slow and hectic periods) means variety in style and quality, but the more consistency to other (believed) authentic rifles the better.
Also, is their a number near the wrist or anywhere on the stock, a simple single digit number, typically near the wrist? (while looking photo the wrist area, there might be remnants of a HZa acceptance)