I spoke with Markus Hofmann, author of the book
Fahrzeuge der Schweizer Armee (vehicles of the Swiss Army). He also wrote a short
article about Fargo 4x2 and GMC 4x2.
Unfortunately, he hasn't found anything in his documents about this particularly image. The Swiss Army bought before the war usually a small quantity of vehicles for test purposes. This GMC could be one of them. It is unlikely that the picture ist dated etc. At this time before of the war most of it was top secret.
I guess the data behind this image woun't be discovered
Greetings
Chris