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Thanks for the help, I'm guessing you and Butch meant the 2 screws holding the arms in place circled in the attached pic?
I loosened them but they appear to not have anything to do with the attachment of the 3 upholstered bolsters.
I think it's just the end support for the entire seat section.
Upon further investigation (careful prying!) It looks like the plastic seat back is attached to another while plastic backing underneath that may be used to attach bolsters
Anyway I took my pics to a good local marine upholsterer and he is going to come down to the dock and check it out next time he's in our area