Thought i would clean this up...
$2250 is way too much, especially with these pictures... for a rifle this complex, you really need more & better pictures.
I assume the stock acceptance is e/33 and e/46, this and the manner in which the stock is marked implies this is a production stock (meaning it is normal production, not a rework or re-barrel or a Simson supplied ordnance stock), generally falling in the range of normal Simson d-e block production; this would mean Simson cycled recycled receivers through normal production, which to date no evidence suggests they ever did this, nor would they, they were ready suppliers of ordnance spares to the ordnance system, not "users" of that system. (They supplied spare receivers, not used them - Simson by 1928 had the capability of making 6,000 rifles and 600 MG's a month and this could be increased to 15,000 rifles and 15,000 MG's with the proper resources. This an IAMCC report). There is no reason to believe Simson needed or wanted to recycle receivers or make more rifles than they contracted for and everything suggests Simson made rifles at a steady deliberate pace, far below capability and probably less enthusiastically than other commercial firms would have. Which is probably why they were chosen, probably the smallest, most able (in house) company among those that made the G98 during the war and probably the most loyal to the new Republic (they certainly weren't reactionary...) Seems unlikely they would recycle illegal rifles or receivers.
I am not a "Simson did reworks" (or salvages) advocate, they may have in rare cases, but I see no reason why they would in general and no example has ever been documented... The Germans possessed ordnance facilities very early on, we know this because the IAMCC complained about them, later they grew in number, first Kassel and Spandau supporting the two commands the Reichsheer established. Later Königsberg and there is evidence that smaller operations existed in a number of former Imperial era facilities, mostly in the east where all the action occurred or was expected.. the German report that details the evolution of the Kar.98k, the path taken to develop this shortened Kar.98b (for that is all a Kar.98k is, a Kar.98b shortened six inches - as John Wall once so aptly described it, indeed that was what the Germany Army experimented with in the late 1920's), states that Simson was deemed undesirable because they would cost too much to do the work of conversions, and that most of the Reichsheer ordnance facilities would need modernizing to participate in the program. These experiments occurred in 1929, but this rifle if as advertised would probably date earlier (by stk acceptance and BC - but not the FP which is post-Simson) around the late d-block of Simson 98b production.
Without the RR and clear pictures of the acceptance stamps little can be said with certainty, but a few relevant pictures would go a long way to explaining what is shown. For one if this is a 98b stock, the buttplate would have the serial w/suffix, further if Simson did recycle a receiver the RR would offer evidence of its utilization. While the barrel roughly correlates to the d-e block of normal production, the FP is absolutely wrong for any Simson work. The barreled receiver is simply wrong for the stock... Pictures of the RS is also a must for any evaluation, while it is highly doubtful to date or connect to Simson in any case (most Republican era rifles have subsequent RS updates) it may offer clues to the re-barrel's actual installer (which wasn't Simson, whatever this is, Simson did not FP that barrel..).
Something like this would have to be analysed part by part and just the barrel FP issue alone is enough to exclude it from the stock mating connection (they were not done at the same time). More than likely this is a re-barreled Spandau/16, that caught the eye of some astute and semi-knowledgeable collector, who had a 98b stock that he thought would go well together due to the Simson components.