trans decode

[Mr70]

P-Muncie plant.
9-1969,Last digit of designated vehicles model year,not the actual year it was assembled,which if it was,would be for a 1970 vehicle.
P-September,month assembled.
18-Day of that month.

Sounds like Sept.18th,1968 was the actual assembly date & with no 1st gear like (M-20),B (M-21),C (M-22),suffix designation,was typical for that time period.
1969 model year was the year they were "supposed" to start including the 1st gear ratio notation,but some early assembled examples have clearly shown they were a lil slow with that in the beginning.
It's possible it could be an Over The Counter,singly sold to the public Transmission.
If it was a warrantied service replacement Trans.,installed @ the dealership,I think it would be showing a code first of C9T or CT9,...for Chevrolet-1969-Transmission,and then a 5 digit sequence reference number after that,between 35000 to 44999.