I checked 2 Army Field Manuals, FM 23-8, dated 1974, and TC 23-14, dated 1969 about the ART scope, and neither mentioned a brand of course, but said that the scope was a high quality commercial scope. My research is 30+ years old, and from memory so I may have been mistaken about the brands of scope used for the ART system, and which order they came in.
The 1969 manual is a sniper training manual, and the weapon used is the M-21 with the ART scope. FM 23-8 is mentioned in this manual as well, so earlier issues than 1974 had to exhist, but I do not know if the M-21 was mentioned in them. There is a section on the M-21 system in the 1974 manual.