It is commercial alteration. In the early 1970's many M1 rifles were altered to use front M14 parts. When the M14 op rod was used, the bolt had a roller installed.
The rifle has M1 Bolt, M14 Op Rod, and complete M14 barrel assembly (gas system, sight, flash suppressor etc.). This version remained the 8 round clips, not a magazine.
I have one of the rifles from 1972.