Middle-era 25D4 distributor with no groove for an O-ring, note that early and late 25D4s probably had one. Note that the groove in the drive dog, or between it and the distributor body, is definitely NOT for an O-ring as some have claimed. The drive dog rotates when the engine is running of course, and if the housing extends down that far the O-ring would be ripped to pieces - or do absolutely nothing if it didn't.
I say no groove, but there is a shallow groove right at the top of the shaft, almost concealed by the clamp plate. That is too high to go down inside the housing, and so shallow that any O-ring would simply be pushed up the shaft if the housing was proud of the crankcase. It might be that a thicker O-ring would work in that position. Compare this with the groove and O-ring on the 45D4 ...
... where the O-ring is almost completely in the groove (so to speak). It looks like all 45D4s had one.
Original 41228 for an 18V581/582 which didn't originally have a slot on the left, remanufactured unit on the right with the slot.