No such thing as a 73 122. What is the correct year?

There are three main types of gearbox: M40, a plain 4 speed, M41, an M40 with overdrive, and M410, a reinforced version with overdrive. All of them are to some degree interchangeable.

There are two types of overdrive unit that were used. Early (B18 cars) used the D type, B20s got the J type, which is bigger and stronger.

The M40 and M41 exist in early and late versions. The new type was introduced for the 73 model year 140s, but I'm not sure if it was also used in the 1800ES. The late gearbox has extra reinforcing ribs on the outside of the case, and a lower first gear ratio. Some other internal components are different too. As a complete unit, any M41 (J) will directly replace any other M41 (J) though it may be necessary to change the output flange (they come in different sizes) Internal components are not interchangeable between early and late.

Internal parts can be swapped between M40 and M41, as long as both are either early or late type.

The oil fill hole is at a different level on M40 versus M41, otherwise the cases are the same.

The M410 was originally used on 164s. Some 1800s got them too. Only the overdrive unit (J type) is the same as the one on an M41. It is possible to replace an M410 with an M41, though you may need to change the driveshaft if the length is different. (not sure if it is or not)