I think this is a bus from the so-called HJ-Wanderausstellung,
a travelling exhibition for educating about the HJ (Schaffung
der HJ). Officially the name was Reichsautozug der HJ. It is
noted the busses had a silver-grey color. The sides could be
opened and other busses could be connected aside to get an
even larger exhibition.
There were also other busses in such way for other organizations,
as the so-called Wanderausstellung Vierjahresplan.