I just got a reply back from Marble support about the Marble tang sight reference in post #7 and it is a stock mounted sight.
Troy Bernson <tbernson@marblearms.com>
2:19 PM (1 hour ago)
to me
Yes, the sight is to be mounted on the stock. No, we do not have any type of receiver type peep sight.
Troy Bernson
Customer Service
tbernson@marblearms.com
Marble's Gun Sights
420 Industrail park
Gladstone, MI. 49837
906-428-3710