Yep, you will see them on lot's of things.

For a second I thought he was talking about and rams. I was thinking that those would make some mean flap actuators for your average Cessna.

It's a Bleeder, Just like the ones on your Calipers for the brakes on your Van. It's at the higest point in that hyd system (where all the air would be)

yes "zerk" thank you.....

Looks like I should start a poll.
I would love to throw a wrench on it and pull the zerk out but being they are over 40 years old I am a little afraid it will break off and then I am jacked.
If its a bleeder, the trim pumps pull all the air out by their selves.
If it grease though, for something internal......
got to figure it out. May have to take the risk and throw a wrench on it.

JUST DO IT !!!!!! and get the grease gun ready sooner or later that pins gunna need it especially with the H20 environment ... i gotta buck $ sayz Zerk

The top pin doesn't get greased. Or at least it doesn't now. The pin in the top of the ram is pressed in. The two angles on each side let the pin move. No grease would go to those pins no matter how much I put in it.

zooming in on your 3rd pic looks to be a sleeve on top where the pins pushed thru ... if its solid and pressed pin how is it being lubed to pivot when you run the tab ??

Not sure if they were worried about that when they built the raceboat 42 years ago. This boat was supposed to last a season or two at most. You may be looking at the original prototype of "rocker tabs" here.
I've only had a couple trim tabs over the years but never saw where any of them needed the pins greased. Just like you don't grease the pins on an outdrive (at least Volvo's). The water takes care of that.

PS no sleve in

Not much help but i know cessna did make hydraulic pumps at one time. (had one fail on a 73 Massey Ferguson tractor)

Expensive sucker too, maybe b/c it said "Cessna" on it.....

I gotta buck on bleeder......remove it and you'll find hyd. fluid..