The 1948-1954 big twins used what is called a “sprocket shaft oil slinger”.
The oil slinger is held into the engine case with a snap ring and forces oil around the sprocket shaft back into case using the rotational force of the engine shaft to force the oil along the reverse spiral groove on the inside of the oil slinger back into the engine case.
There is also an updated replacement for this oil slinger that uses a double lipped seal and is held in the case with a snap ring the same as the “sprocket shaft oil slinger”.
1954-1968 FL models used a “case insert nut”.
The engine case insert nut is threaded into the engine case. and works on the same principal as the oil slinger used on earlier models. using rotational force and a reverse spiral groove to force oil back into the engine case.
There is a double lipped seal and retainer available to update the “case insert nut”. It threads into the engine case the same as the “engine case nut”.
The seals can be purchased separately.
Many 1948 to 1954 motors have been updated to use the 1955 and later Timken bearings and splined engine shaft.
Many 1948-1954 motors have had left side cases replaced with 1955 and later cases and numbers changed to match original year.
The case insert nut FL 1955-1968 and the seal retainer are very similar in appearance. The seal retainer does not have a spiral groove cut into the inside surface. The seals will not fit the case insert nut.