Hello and welcome to NLG.
There isn’t a lot of knowledge base on those machines but there are a few manuals in the file system, link below. You need to be a contributing member in order to access the files, you can make a one-time contribution or sign up for a recurring paymentby clicking on one of the links at the bottom of the page.
Sorry I can’t answer for whether there is a missing board, I have never seen an upright version.
Typically you have a game board and a monitor, not sure what you are thinking is missing. Is there a wore going from the game to the monitor board? If anything you are missing a hopper.
Member Badbaud actually worked for them somewhere around this era, he might have some insight.
Chris