When muscle car enthusiasts refer to Muncie transmissions, they are generally referring to the 1963 -1974 four speed transmissions for high performance applications. The Muncie transmission plant made all kinds of transmissions, transfer cases and even outboard motors. This guide is only about 1963-1974, four speed, aluminum case transmissions.
Borg Warner made a T-10 transmission that was used in Chevrolet's performance cars up until 1963. In 1963, GM decided to replace it with it's own transmission design based on the T-10. The Muncie transmission was used for all high performance applications until the Super T10 replaced it in 1974. Lower performance applications during these years used a Saginaw four speed with cast iron case.
The two options for 1963 - 1965 transmissions were M20 (wide ratio gears), and M21 (close ratio gears). M20 has a lower first gear than the M21. All Muncies have a 1:1 top gear ratio. In 1966 a third option, an extra strong M22 became available. It is nick named the 'Rockcrusher' because the gears have less of a helix to them, so you get gear noise when driving. M22 gears also have a stronger alloy steel than M20 and M21. The M22 was discontinued in 1972.
Muncie 4 speeds have an aluminum case, side cover and tail housing. It uses 7 bolts to attach the side cover (Borg Warner uses 9). 1963 - 68 Muncie's had the shifter levers attach to the transmission by nuts. 1969 - 74 had levers that attached to the transmission by hex head screws. The main case has a casting number on it and can be used to approximate the year it was manufactured:
1963 3831704 (Corvette only)
1964 3839606 (Corvette only)
1964 - 65 3864848 (Corvette, Nova, Chevelle, Impala)
1965 3851325 (Corvette, Nova, Chevelle, Impala)
1966 - 67 3885010 (Corvette, Nova, Chevelle, Impala, Camaro)
1968 - 70 3925660 (Corvette, Nova, Chevelle, Impala, Camaro)
1971 - 74 3925661 (Corvette, Nova, Chevelle, Impala, Camaro)
Of course the tail housing and the side cover have a casting numbers on them as well and all are related to the year of manufacture. If you are really interested in numbers matching cars I recommend Alan Colvin's book, 'Chevrolet by the numbers'.
It is almost impossible to be sure what gear ratio is in the transmission without taking the side cover off and looking at the gears, and counting teeth. 40 years have gone by since the transmission was new. It has probably been apart and may have different parts from when it was new.
Muncie transmission were also used on other GM production lines, such as Pontiac, Buick and Oldsmobile.