It looks it was originally something close to the 1948
model (Liner?) that the Cooper couple had at the vintage rally here.
There is a pic of it on the vintage rally thread that either Pahaska or PeterMH posted. The rear window looks exactly the same.
That must have been one handy hippie or whoever built it.
The seats could be stand ups, sorta like what you used to see in old Divco milk trucks, my dad had a Harrington-Marmon Delivrall that had something like bicycle seats in it, hand controls as well if I remember correctly. doubt if any of it would be legal today.