Recess on one part of a sliding joint yoke

Moss GUJ115 with a short nipple

Olphartz GUJ115 with a very long nipple. There are also some suppliers with a 'medium' length nipple.

Looking closely at the bulge in the casting that the nipple screws into in the two above, it's apparent that the Moss is 'shorter', so the nipple will be at a greater angle relative to the line of the arms of the spider, compared to the Olphartz item. It may be that with the greater angle only the short one will fit inside the yokes.

Yet another variation - ANG GUJ115 with a short nipple in one of the cups

Bee's new shaft, unlike the old shaft grease nipples on the UJs as well as the sliding-joint, but if changing them you would need to fit the same type and nipple or balance could be compromised. Hopefully not required in my lifetime:

Bee's old shaft has a hole in the end-cap of the sliding joint which means that grease pumped in the nipple comes straight out there:

Said to be necessary to prevent the cap getting blown off if the joint is filled with grease while fully extended. Firstly I can imagine that it would need a lot of force to do that, and one would have to compress it before it could be fitted to a car, and secondly sealed joints (as per my old V8 propshaft) do not have that vent so they obviously 'breathe' some other way.