I have that same cabinet. I purchased it new in the mid 80's. It is a fabulous cabinet and very well made. What you do is bolt your machine on the base. You can move that one piece on the right either up to the machine (like pictured) or down flat for more sewing room. You have to move it up around the machine to close it and put it back up into the cabinet.
I don't remember what I paid for it, but it wasn't cheap. I had a full size Janome machine bolted on mine.