Need help from the German armor experts.
Here are the options I have available for the Stug IV mid I am fittin to build:
Unidentified Unit, East Prussia, 1944
17 Pz. Gren. Div. "Gotz von Berlichingen," Normandy 1944 (2 different schemes)
Pz. Lehr Div., Normandy 1944
The Unidentified Unit vehicle has skirts on it and the others don't. I am leaning more toward the Normandy vehicles at this point.
My question is this: Which of these vehicles (if any) would have zimmerit? I have tried the webs, but can't really find anything.

This kit has the Ausf J chassis if I am correct. That tank entered service in the Summer 44, so with Zimmerit cancelled in Sept '44 few if any Mid Stug's would have had Zim I would expect.

All the GvB StuG IVs I have seen have Zimmerit. Didn't know that Lehr had any - they were mostly issued to infantry divs (of course it could be an error). On my HDD I have a list of recipients so can check in a couple of days if Lehr received any.

The only issue here is that the fighting in Normandy was mostly over by the end of August, so with that in mind the two Normandy options should have Zimmerit applied.
Most Stug IV's were based on the Ausf H chassis.

Two Stug IV's with Zim

Man, this isn't easy. So, from the two places I have queried this, I get that they did have it, or didn't...
In photos, like the ones provided here, it clearly shows zim, but other info says no.
And here is another snag. In researching zim options, I see that ATAK makes a set for early and late Stug IVs, but nothing about mid. If, indeed, mid Stug IVs had zim, would either of those options work? I know very little (as you have probably figured by now) about Stugs, so I don't know if there were differences in the hulls and superstructures between the different variants.

Aargh, the joys of trying to accurately date a german afv. First off, to join the others - a stug in Normandy would have zimm. The Atak early (for Dragon 6520) should fit a mid, differences were mainly internal and on the roof.
I'm thinking you have Dragon #6582, which has 4-a-side return rollers and the large exhaust muffler (like the early). However, it seems to have the roof mounts for the auxiliary crane which were added from the end of June '44 and makes it unlikely to have reached the Normandy battles (at least with the units mentioned) as apparently few replacement vehicles were issued. So OOB it's a July-September '44 vehicle with zimm

Thanks all for the answers.
Richard, with that I think I will for sure be doing the zimmerit. I am going to check out the ATAK set, and also experiment with doing it myself since I have the Tamiya zimmerit applicator set. If the putty experiments go ok, I will probably go with that. I've done it once, but not sure if I want to tackle it again.
Thanks again for the input everyone.

The problem you face is that terms like early, mid, late and final are all ex-post terms that would have had no meaning to anyone at the time because they are so vague (and nobody knew when production would end). It also presupposes that the kit manufacturer has provided a set of features which actually belong on the vehicle you are building.

The StuG IV, being a combination of Pz IV chassis and StuG III superstructure, reflected what parts were supplied by the relevant component suppliers.

The best starting point is to find a vehicle in a photo that you want to use and then use the visible features to work out from the timeline of production mods which features it won't have, which it should and which it might. Books by Spielberger, Jentz and Doyle are generally reliable although some of the early works are now out of date. There is an Osprey New Vanguard on the StuG and also a Panzer Tracts, although the latter is hard to find these days. Neither is (or was when released) expensive. Pretty sure that GvB had the only StuG IVs in Normandie so that limits you if opting for that campaign.

Given as zimm application ended ca 18 SEP 1944.... EVERY StuG IV seen in any combat area before perhaps mid Oct would have had zimmerit applied. Conversely, any StuG IV that left the factory after mid-Sept '44 would have left without zimmerit. As others have noted, there is some further complexity added by our use of "early, mid-, late" production - these are not actually very meaningful. German vehicle assembly practices further confused things by using a "last in - first out" approach to parts-management, so the newest parts could be found attached to older hulls, and new hulls left with old-stock old-version parts. "Versions" were not as meaningful as some wish!

So, if modeling a StuG IV in service before mid- OCT, use zimm. After mid- Oct, your Stug would have been increasingly likely to be zimm-free as you get further and further into '44 and '45 - but "older", zimm'd StuGs persisted to the very end. This means that you can legitimately model a zimm'd StuG IV active in May 1945 (as long as its features other than replacement wheels and such are consistent with those being put on through Sept '44).

Probably about half of the 1100+ StuG IV produced between Dec 1943 and April 1945 received factory zimmerit, so you have some range of vehicles to choose from as you go!

Bob