Here's some shots of the planetary assembly...
Try as I might, I cannot find even the tiniest of groves for an O-ring to fit into other than the groove for the snap ring (part 25, in the diagram I posted earlier) above the pin hole. Maybe this model is one of those units that omitted the O-ring (as you previously mentioned). A vernier caliper could not locate an O-ring groove either.
The serial number of this unit is: WR5059395
Getting back to the images, the planetary assembly is the only part I have not disassembled yet, and it's not much cleaner, now, than it was when I first removed it from the mixer housing - meaning that not much grease leaked into the planetary to begin with.
EDIT:
I beg your pardon, I meant to say I used a vernier calipers to make these measurements with, but I did recheck the shaft with a micrometer, and the shaft uniformly measures .498" OD from top to bottom with some variation in the tenths range (.0000). The pressed-in sleeve bearing that the shaft fits into measures at .500" so there is not much room for play.