I found it (took a while) but it was a major disappointment.
The picture can be found in 'Beute-Kraftfahrzeuge und -Panzer der deutsche Wehrmacht' by Walter J. Spielberger on page 107.
It belongs to an article about Holzgasgeneratoren. Not a clue what that is called in English. Having a vehicle run on the gases produced by the burning of wood is what it does.
The thing on the back that Phylo pointed out is the generator.
There is however nothing about the vehicle itself!! Well...apart from mentioning that "apparently" such vehicles were used (also with Pz IV turret) to train the three man turret crews.
I checked on AHF (and Tom shouldn't be so surprised, there he thought this vehicle had a thyroid problem
21 april 2005) but the answer was never found there either.
One member says he has a picture of the same vehicle but with a Pz IV-turret. The link he provides is dead though
In another book I found something else that might point us in the right direction
There are no pictures but in 'Die Panzerkampfwagen I und II und ihre Abarten' also by Spielberger on page 33 it says (in a short and loose translation) that in 1934 Rheinmetall started the development of "3,7cm Selbstfahrlafette L/45". The Leichttractor chassis was used with a 'new turret' wich mounted the 3,7cm Pak L/45 and an MG. The vehicle weighed 8.5 tons, had a maximum speed of 35 km/h and 13mm front armour to protect the three man crew. The vehicle was never introduced to the troops.
Rheimetall also started development of what was to become the Pz III turret in 1934.
I think that this may be the same vehicle as in the picture even though there are a few contradictions. Maybe after sufficient Pz III's were available for training purposes (never introduced to the troops??) these vehicles were used the test the generators.