Early SM-420's don't have a front seal- - - -they just have a reverse spiral groove that pushes gear lube back toward the transmission case as the shaft turns, and an oil slinger that spins ahead of the input shaft bearing. If the case is too full and the vehicle is parked in a steep nose down position, lube will leak into the bell housing and contaminate the clutch lining. If you can reach the lube level with a fingertip through the fill hole, there's enough there to keep the gears and bearings happy.
Jerry
