never seen it in all my 8 years of yarding for M body parts, nobody's ever documented it on dippy.org, and there's no reference to 11.75" discs in my '85,'86, & '88 M body FSM's....if I had to guess, it was a popular upgrade for people after the fact....

were they pin or slider mount? I've only seen slider mounts in factory apps after about '77 or so on all the late B &R's I've scrounged through, too...