If the only code on the BODY plate is M20 it will have been a 253 4spd with 3.08:1 diff. You'd be surprised how many V8 4spd bench set HQ's were made. It appears they were a lot more common than the same combo with buckets outside of XX7. I've only ever found 7 x bucket seat HQ commercials that were not Sandmans, of those 5 x 308 and 2 x 253, 4 x M21 and 1 x M20 (the other two are too early to know what box from the ID).