I spent quite a bit of time during the late 60's early 70's putting rounds downrange over NM courses through Navy 7.62 Garands on various teams and never experienced an M1icon Thumb. AND I AM LEFT HANDED! I also ran my Dad's for a while (before it was stolen) during the early 60's and did the high school & college ROTC bit (they had free ammunition for team members!!) including a summer at Ft. Polk, LA without ever having a problem. So in my mind, it is a nonexistent issue.
Just my .02 YMMV