field mod?
D-11 introduced automatic water/methanol injection- previous models supported it, but it wasn't automatic.
D-15 introduced the wing racks, which were also made wet for drop tank use. The Paddle prop was also installed at the factory at this point.
Field mods for the razorbacks focused on turbo-supercharger wastegate manipulation- this was done to increase the boost to the -2800 up front, supplying more power. Widewing has a great post in here regarding an interview with Bob Johnson. He talks about it, and brags about his Thunderbolt and what she could do.
When 150 octane fuel was introduced, they could increase the boost even more to the tune of 78-80" hg.
Paddle props were sent to the theater with mod kits, I know that. Basically, at the end of it- all Jugs in Europe were toting the paddle blade.
I don't know about wing rack mods. I know some were removable, but as to which models I really don't know. Widewing would know more.