DUKW from WWII is 33' long, GPA amphibious Jeep looks nothing like this either. I think it's one of the experimental models that come up once in a while, I would gladly buy it if it were near me in Virginia and you had no interest.
V/R W Winget
Upon detailed examination of the photos, the following is seen: Some Mule components used, but it's NOT a mule. The organization that built it likely stripped a MULE for it's drivetrain, hence the serial number plate and the shifting pattern. The steering wheel looks to be off another Mil vehicle like an Ontos possibly. The starter casting on the engine is broken off. The elaborate 4 pipe stainless steel exhaust may have been conceived of for preventing a fire or such, but is really overkill. Parts inside the tub may or may not be useful or from this vehicle. My bet: prototype, developed for testing at Aberdeen, MD, used in swamps and such for years to go check out weapons effects or watch the bay banks for clearance prior to large artillery shots, etc. Not home built, many pocket compartments along the sides of the inside hull like a radio mount or fuel tank bracket setup. Even the add on mud chains on the wheels seem set up just for this rig. Windshield folds down (maybe) and it had a full enclosure, possible for winter testing, which may also be why the large exhaust might have been used as a hot air source into the cab.(that or it was a Hunters addition) Can't see how people would ride in it without decking, it may have not progressed enough before sinking once or getting held up in mud due to weight and being parked.
MD. Inspection ran out in 1984, you can bet that's around the last time it had real use, check the license plate on the windshield to see what your DMV states, but it would be registered as the mule dataplate VIN.
Neat experimental vehicle, there were a few amphibious MULE attempts, it's not a COOT (another articulated amphib) but looks like an interesting experiment, likely with bilge pumps, hence the round cutouts for access plates.
PS: Ice in the plastic bin means most of that stuff may be only good for patterns.
W. Winget