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

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!