The "X4" 400 motor is a 1973 400 170/185 BHP (2-Bbl)
I'm thinking that the four speed is a Frankenstein.
The main case is from an M20, built on August 6, 1964. That car it came from was built in the Kansas City plant for 1965 production.
3857584 is the tailshaft housing for a 1966 – 1970 (Passenger side speedo), 27 spline output shaft, meaning it was put onto the main case when the trans was rebuilt.
3884685 is the side cover issued with "584" tailshaft housing units with stud type shift shafts. (As opposed to bolts.)
It's possible that the trans had a bad case and was re-built using a new one.
All bets are off as to whether it is an M20 or M21 until you break it open and count teeth.
