Thanks for your reply, BertBert wrote
a very interesting question about the Mercedes-Benz type G 3 a. You are right, it is a subvariant of the Kfz. 62.
It is a vehicle of a Beobachtungsabteilung (have a look on the invitation of the Beobachtungsabteilung for a stew lunch). It cannot be a flash-light analysing motor vehicle or a Wetterkraftwagen – weather motor vehicle or a print shop motor vehicle, because these vehicles have another superstructure. So it could be a Stabsauswertekraftwagen - staff analysing motor vehicle or a Schallauswertewagen - sound analysing motor vehicle. Here a pic of such a vehicle (source: Handbuch für Heerestaktik des Lehrstabes für Heerestaktik beim Reichsministerium der Luftfahrt 1939).
I think that there are also other two possibilities: Kfz.62 Vermessungsauswertekraftwagen (surveying evaluation motor vehicle) or Kfz.62 Schallaufnahmekraftwagen (sound recording motor vehicle) ...

Regards
Sturm78