Thanks. The seller didn't help himself, under-advertising as a "Mauser Type B with Peep Sight".

After poring over/through Speed's book again tonight I found something interesting in the "Special Range Rifle" section, a $25 option listed in the text of the Stoeger catalogue description (page 176, plate #351):
"No. 621 - Same as above, with collapsible peep sight fitted in square top bridge"

That ad ran some 20+ years after this rifle was made, but seems to lend additional evidence that this is factory work (along with the Kurz mentioned in the other post).