Hmm. So much for no field mods in the game then huh.
It wasn't a field mod, but something done on the way to the theater. This was true of all combat birds. Things were tweaked at the depots before they went to their units. The 5th AF for example had ball turrets removed from their B24s and tunnel guns in their place as working from wet and rough fields caused the ball turrets to get covered in mud and be rendered useless. The B25 mods were done before being assigned to squadrons, again as that was the requirement for 5th AF. Long range "Tokyo" tanks were added to many 8th AF B17s at the depot's in theater.
A field mod would be a one off or limited number of mods done at a squadron level or by an individual pilot or ground crew. The only field mod I can think of in game, is the Malcom hood on the P51B/C as that was produced in England and added as they became available at the group and squadron levels. It never made it to the MTO or PTO that I know of.