The M is Mound Road assembly plant.
The R is for Regular fuel recommended.
2887 is the final assembly date from the 10,000 day calendar - June 23, 1969.
On the passenger side down low is a flat pad, about 3" long. And letters / number stamped on it?
Based on all the hardware store bolts and nuts in the photo, this engine could have been put together with parts from all over?
