The way the seating in my boat works is as follows:
I have 2 Swivl Eze brand stanless steel deck plates mounted aft of my center console. They are thru-bolted in the deck.
Into these deck plates I have the option of putting 1 or 2 barstools. the barstools have the following fixture mounted to their bottom:
And the leaning post/rocket launcher I fabricated has 2 of them mounted to the bottom.
The Barstool uses one of these from the deck mount to the the fixture above
and the leaning post utiulized two of them.
The deck plates and the fixture under the seating is stainless and the posts are aluminum
All of the above pop on and off in 20 seconds and are all easily dissasembled.
Again my boat is under wraps so I do not have shots of the deck plates but I should be able to conjure up some photos of the leaning post and the barstools.