The 879...is a permanently sealed gimbal bearing. If your gimbal hsg. has a pipe plug in the lube port on the right, this is what come out.
I'm a "more grease is better" guy, and I've found permalube U joints can take more grease than what is provided, and I guess the permanently sealed gimbal brg. is the same. I have the previous rev. Gen II. It has the greaseable version which I like. When I do my annual/semi annual O/D pull, I give the bearing a few extra pumps of grease to force out the old grease so I can check for water, and repack the thing.
Merc replaced their greaseable gimbal brgs. with the above and a pipe plug to replace the grease zerk. >$100. I suggest using the Sierra18-2100, remove the pipe plug and insert a grease fitting if you don't have one. $$48. It's the same Link-Belt part that merc. uses, and you can assure yourself the bearing is fully packed, always.