It's the ever-popular Horch and Flak 38!
Nice dio and very interesting!
I did one of these things a couple years ago using the Tamiya Horch with a Tamiya Flak 38 and a bunch of scratch work - loads of fun! It's always very interesting to me to see how folks interpret this somehow "classical" field-mod combo. There have been a range of "gun-deck"types done (wood, like both you and I have done, steel plates, larger and smaller areas, sunken down into the hull - again like you and I have done -, set up on the top, etc.) and a couple of different gun-positions used.... you have elected that arrangement with the "single" arm positioned forward and between the front seat-backs, whereas I went a different route.
I generally associate these conversions with the DAK and the Italian campaign, though there is some suggestion that they were seen here, there and maybe about anywhere the Horch and gun showed up - which would be about any front the Germans found themselves in! Your's is the first build I've run across a build depicting what looks to be an earlier-war Euro-setting with an all-dunkelgrau specimen - not that I have seen every Horch/Flak posted... if nothing else, you have carved a somewhat different niche for yours!
You didn't mention whether you used any PE or such - evidently you did swap in a metal barrel (though I would guess that the JR barrel was intended for a Flak 30? It looks to be for that gun, based on what its muzzle-brake looks like. The Flak 38 barrel has/had a conical flash-hider.) - and there look to be a few non-Tamiya bits here and there!
What did you use for your gun-deck? Looks almost like a ship's deck or maybe some sort of siding panel?
In terms of critique; aside from the wrong-looking gun barrel, and the more stowage you mentioned, I suggest dirtying-up the deck and truck some - they look a bit too clean to me - the more so as your trailer is a bit dirtier! Simply brush on some dry chalk dust or pigments and it could get dirty fast! A more difficult fix would be to add on the framed-net shell-basket typically seen on Flak 38 (but us modelers love a challenge, right?!).
By me, the Tamiya Horch is a classic and I found it fun to trick out a little. Hope you found yours was fun, too - it looks like it should have been!
Here is a deck-shot of my effort to compare similar approach on the wood deck, different gun setting - note the barrel (RB Flak 38 L/65) and the shell-net (scratched from sprue and bridal veil material - a little tedious...!).
So... Welcome to the Horch flak-truck "club"! Nice job, Pard!
Bob