Agreed, some sort of mounting gadget, but not factory.I think that was some kind of home brew thing . Magma shows nothing like it on their site .
That would be my guess as well. I've got the current rod holder mount for a power boat and it has the same bend angle as in the OP's photo.I think it might work in a rod holder, with a peg in the bottom that usually catches the grove in the bottom of a fishing rod.
You'd twist to grab that peg, then tighten that plastic thing. Maybe they used to make it that way??? but the current rod holder mount is different.