May be legit
As Wayne has mentioned the circumstances from which this gun surfaced give some creedence to it having been captured in this configuration.
Also, three other guns using what appears to be this same adaptor have surfaced during my 35+ years of observing and studying them.
There is also one of these mounts that Bob Simpson acquired uninstalled in Switzerland last year.
I initially thought it likely these were post war creations.
I now believe these were prototype Sniper Trainers BUILT POST PRODUCTION and not at Mauser.
Richard Law presented documents in Backbone II that indicated Weihrauch had developed a mount for the DSM and was in possession of a prototype. Also, another type I am confident is attributable to Weihrauch as it is marked with the WaA359 acceptance stamp. Essentially that mount is set of high rings and bases which attaches via new grooves cut in the barrel between the rear sight and receiver.
There are likely other variations which are legit done in some quantity as prior to the SU27 altered Gustloff KKW there was no standard scope mounted .22 trainer. There were some trainers modified for use at sniper schools and that is fact. What they looked like is not known. What firms or gunsmiths or Ordance Shops did the modification is not known outside those Gustloff KKW's. This is high on the list of things I will be searching for evidence on during the upcoming trip to Meiningen, Suhl and Zella Mehlis. I am hopeful to find some paper trail regarding the Weihrauch effort and may point to other documents if that correspondence has survived.
Such a gun will always remain controversial. The provenance on any sniper is foremost. I assure you there has long been, and still are people in the U.S. who are capable of very high quality fakery that is un-detectable to most people.
From my perspective, if I were acquiring it under the circumstances it supposedly emerged and at such a price, I would feel pretty good about it. On the other hand, at a Show, collectable shop, or from another collector whom I did not know, I would not touch it with a 10 foot pole.
From some dealers I would not touch it if it had Hitler's fingerprints on it and capture papers.
To be clear, I do not think this is Mauser made prototype but it is feasable. I think from what I have seen regarding procurement of .22 scoped trainers for the sniper schools these were altered post production in numerous fashion and small quantity. This particular mount may have been made by one firm and used by several smaller firms or gunsmiths (likely in either Suhl or Zella Mehlis) on contract for the sniper schools.
Not much help, just my take on it.