My best guess is an adjustable mount for an instrument display... but, yeah, a little dumbfounded myself.
I have a similar box of things passed to me from the previous owner (a close friend) which was passed to him from the previous, previous owner. That box lives in my brother's garage and is not to be opened for fear of creating new projects and/or deep speculation.
Why, that is obviously what it clearly is. It is for the purpose for which it was so well designed. Many boats have such devices, which serve many purposes, although of various manufacture and materials.
I use one of those on my boat to hold a block of cheese to the floor. The plug is useless, it has no wire.