Ref: the Vietnam era XM21 ART 1 Sniper Scope...I sold it at the beginning of the year, and for what I was asking: $3,800.00.
I did have an ART 1 Mount Only some 30 years ago - it's on an early Springfield Armory, Inc., that was actually worked over by USAMTU Armorers in Fort Benning, Georgia...the good old days when it wasn't a big deal to move long guns across the border.
So the mount is still out there, somewhere. I have never seen the rifle here on CGN. Would be easy to recognize by the 1970's period USAMTU camo paint job.
Cheers!