Mark,
Courtesy of Andrew's great 190 site, here's a 190A-5 main plate, W.Nr. 1595.
http://fw190.hobbyvista.com/polyak6.htm
My experience with 190 plates (make that plates in general), indicates that manufacturer letter codes (denoting primary builder, license builder and/or builder factory site) were not consistently used. This applies to main airframe ID plates as well as airframe component and subassembly data plates.
Here you will find examples of each type.
http://fw190.hobbyvista.com/plates.htm
Am not sure this answers your question, but variability is common when it comes to entries on plates, especially hand entered information, even at the same factory, across production blocks.
Steve