If it were off a P-type you could fairly safely say that it is an anchor plate for the bench seat recliner quadrant, but an M .... (scratches head) .....
So it probably isn't an M part - I didn't think so. My first guess on it was the seatback stop anchor for the top of the seatback in a J or P. The threaded rod is 3/16 whitworth (1/4" diameter). Does the seatback stop use this thread size?
Tom, the thread is right for the M type seat back adjuster but the bracket is circular and aluminium and has a female thread, the stud being on the part that bears on the seat back. Keith