Hi Rick,
The question wasn't hard and like Bob said field mods were a nightmare. The bad news is that field mods were almost never photographed or documented, therefore we have so little info about them. It's the same with Krupp, who built the StuG IV. Existing documents and photos are far and few between. Most got destroyed in air raids or disappeared after the war. There are some, but it's just like Panther production. MAN has a full array of photos shot by the MAN official photographer, but little of the written documents survived. MNH has lots of written documents, but very few actual photos of production and such...
One day and we can only hope, these missing (if they weren't destroyed) documents and photos will surface, may be de-classified and be published. The Bundesarchiv here has a lot of info, but seeking it out is a very difficult and time consuming task as there are hundreds of thousands of photos to go through. Doing a search by name only gives you about a 5% chance of a hit on the subject. I haven't delved into the NARA archives in America yet. They have a lot of photos taken during the war, including the MAN and MNH factories after their capture. Again, I don't believe they have de-classified a lot stuff taken from German armament works after the war either. I could be wrong…..
Bleib Gesund
Allen