It's something that pops up, particularly in early war stuff when they were clamouring for rifles. Jordan has an 07/14 Danzig Gew98 and there are several others known with similar circumstances.Is there any hunches as to why this receiver would have intially gone unused?
Reasons for lack of initial use could have been previously failed parts that weren't re-inspected until later, surplus production, misdirected parts, who knows? What we do know is it occurred and they are cool to encounter.