Dear Mike,

according our today´s knowledges the "second" plate showed modifications ("Anderungen") of airframe. Modifications came during a production and it was needed to noted them on airframe from a reason of possible reparation of plane (it had to be noted, the concrete airframe contents some modifications). The modifications were noted on the plate in a form of numbers only and every number (probably) meant some concrete modification. We don´t know a list of the modifications and numbers, unfortunately.
The modifications were mostly a construction, e.i. changes in a strenght of airframe, internal changes, etc. I think, they weren´t visible from outer view.
Due to, some plane wore the "second plate" some not.
Please, note, the special problem isn´t clear yet for me and I hope to get more information about it in a future.

Warmest regards

Tomáš Poruba