Brian, I am far from an expert on this vehicle or this gun, but the FlaK 36 and FlaK 37 were quite similar. Without looking at the kit and the gun close-up, it's entirely possible that Trumpeter has mislabled the gun as the 37. Spielberger does list the FlaK 36 as the gun for the 6/2 in Halftracked Vehicles of ther German Army 1909-1945.
It is also possible, as you allude to, that the Wehrmacht simply mixed and matched available guns with platforms. Without knowing whether Trumpeter designed this kit from photos or measured off a surviving vehicle, we can't know definitively what they were thinking.
Perhaps someone with an intimate knowledge of the two 37mm guns can look at the photos and illustrations and determine whether Trumpeter has mounted the late 37 or has mis-labled the gun as the later model? As I said earlier, the differences between the two are mostly in the sight from what I know, so this could very well be a 36 and is just labeled incorrectly.
I do like the inclusion of a trailer and extra barrels. These guns (like the quad 20) tended to burn up barrels that then needed to be swapped-out.
Finally, both the 36 and 37 were stages leading up to the FlaK 43 version.