Dennis Quella has sold that design U-joint for years! There is a small plunger as part of the fitting that pops up to release the pressure caused by pumping in too much grease during service. I have installed many of them into stock half shafts and IMHO they are among the best joints available for the stock shafts. Why in the world would you remove the fitting and plug the hole?? It is there for a reason . If you overgrease the joint, it gives the pressure surplus grease somewhere to go instead of blowing past the rubber seals. You can also "flush " the old grease out of the joint without damaging the seals.
Ron