missing oil seal

[John Cave <cavej@mcmaster.ca>]

I,m replacing a won out bearing in the rear extension of my 1500
transmission, while i,m down this far i thought i,d replace all the
seals and bearings, Rimmer,s shows an oil seal between the bell housing
and the gearbox,i have what looks like a left handed thread, in a large
steel insert in the bell housing, is it likley this a reverse oil
slinger and is replaceing a rubber oil seal my car is a 78 i dont know
what year the transmission is ,it is a single rail.

                             Thanks John Cave