When the work was done is a complete mystery. It has all the looks of a professional conversion from back in the day. The cabinet work and "aft stateroom" appear to be original to the unit, and are still in very good condition in finish and construction. All the 120VAC wiring looks professional, right down the the slimline electrical boxes in the walls.
The toilet/shower seems to be a later addition with later materials, but done well. While I cant be sure, I think they took up a bit of the living/dining area for it, or perhaps there was more storage in that area.
Underneath the more modern flooring the is painted plywood on top of the aluminum deck. Behind the knotty pine plank beadboard walls is a god-awful forest green paneling made of pressed board, like pegboard material.
History, I only have what the PO gave me. As best he knows, the machine started out in AZ, where the upper rear window and bunk was removed in th aft stateroom {lol!} and a window AC installed. The Grumman then made its way to WI, where the AC was removed and a steel panel installed. Here the rear bumper was extended 2ft and a motorcycle chock welded up. This owner was known to haul a Honda Goldwing {naked model} and outboard boat to vacation spots.
Then it went to Arkansas for a few years with a new owner, who used it for shelter at first, then storage. He then moved here to Charleston SC, where he advertised it on Craigslist.