I suspect this extended bed was an owner mod, as it’s longer and more permanent than the commercial extensions that were produced.

https://dallas.craigslist.org/ndf/cto/d/aubrey-1945-gpw-willys-jeep-wwii/7637095626.html

“1945 Ford GPW Military Jeep
More hard to find than the Willys models.

Runs good and has very rare bed extension I was told was for use by MASH units. Never seen another one with this accessory

12v, Winch, runs and drives, high and low range work fine, winch, tows easily with attached towbar and new tires.
Last year made

Restore or drive or have the perfect post apocalypse rig/hunting truck
Clear title

Cash of possible trade up or down for crew cab pickup or good running 60’s era survivor (un-modified or hot rodded) sedan, nice hybrid…don’t laugh I got a long commute…or maybe something interesting. Prefer cash. No checks no payments and no bs”