The black stuff is assembly paste.  Final backlash, after fully tightening, was 0.004", which is 0.001" over tolerance.  Considering that this is a new pinion with the original drive gear the slightly loose tolerance is more than acceptable.

The other end of the drive shaft.  The bearing is fitted to the shaft before knocking the bearing into its housing in the crankcase.  Care is necessary to ensure that the gear teeth mesh as the shaft slides into position.

The bearing retainer bolted into place.  The elusive forcer ring is shown resting on the shaft prior to fitting the gear.

Water pump drive gear - rear face.  The rebate holds the forcer ring, shown below.  Both parts are in the same orientation.  When fitted together the rear rims are level and clamp against the inner bearing race.

The original forcer ring had gone AWOL in the distant past and its absence caused some difficulty during removal.  As its name implies the ring acts as a buffer against which the extraction bolts press when removing the gear.

The top four wheels in place.  All bearings were lubricated with a few drops of engine oil mixed with assembly paste.

The water pump drive is incomplete because the retaining nut, which was a non-original part, was found to be damaged.  The search is now on ...