Only 2,138 16" heavy duty buses were built, before production was wound down in 1992.
The 16" version differs from the 14" in a lot of details. Thicker or more rigid material for the structure and suspension, different transmission ratios, with a rear diff-lock included in the package, and for some countries a front diff-lock as well. Special air tubes for the transmission to avoid to sucking water while wading, underbody protection shields, enlarged brakes and a modified suspension which enlarges the wheelbase another 20mm for clearance.
More info can be found at vanagon.com
The interior is obviously very spartan as Prisoner types don't normally have need to stop off for a cuppa on route. In contrast I do, regularly, so basically I needed to find an interior a little more luxurious.
Anyway, if you are going to do something you might as well do it properly. To this end I splashed out on a used but original Westfalia interior. Fabulous. Rest assured that any prisoner now carried, is in utmost comfort. Click to see pictures full size.