The majority of U (Umrust-Bausatzen) or R (Rustsatze)designations are merely test beds for the next in the 190A series. What you will notice is as the series of 190A's go up in number its loadout of weapons or equiptment will have been tested on the previous 190A model. For instance 'Fw190A5/U7 Armament test bed for two external 30mm MK103 cannons or two internal 30mm MK 108 cannons.'
A 190A5 with 30mm MK108s in the wing is the precurser to the 190A8 with that same armament. (You may have the odd engine upgrade too as it changes from A5 to A6 and so on).The shame is that most of these test beds are single aircraft or very usually a few solitary aircraft, and sometimes they are not combat tested in this form but just evaluated until the weapons are accepted into a factory produced 'new' series and is then used in combat.
Interestingly our 190A5 has the posibility to match a loadout for a Fw190A5/R6, which isnt a factory produced loadout but a field conversion(Rustsatze), This 190 would be as follows;
Fw190A5/R6 Installation of one Wr-21 anti-bomber rocket tube beneath each wing(similar to the Fw190A4/R6) outer wing MGFF cannons were removed.
The 190a5/U9's 13mm MG131 werent 'factory' introduced until the 190A7. Even a 190A6 would still retain the 7.92mm Mg17's if it was a factory produced A6.Some 190A6s also were tested with the 13mm MG131 of course:).
they like to keep it confusing eh?
