[RalphZimmer]

Gentlemen,

concerning the second data plate I looked up JaPo's Messerschmitt Bf109G-10/U4, which contains photos of both types of data plate. I did not see any "first-hand-source" so the following are just my SPECULATIONS:

Firstly, "Änderungsstufe" literally translates as "degree of changes".
This MIGHT mean, that a "mark" for every change could be added there. As this photo (Japo, page 9) shows only one "mark" (12.44) this might be a date. This date, as stated earlier in this thread, might show the date up to which all changes are included.
Remember, that even after finishing the design of a special subtype the design work went on. As seen on the G-6 variant one might see new sub-assemblies like canopies, rudders and so on - but the a/c is still called a G-6.
My theory is that these "changes" might be changes in the design of a special subtype, which were incorporated during production.
An a/c produced around -say - October might thus have no second data plate, while one produced in December 1944 has one with one entry.
An a/c produced in March 1945 might have more of this entries.
In that way one is able to differentiate between a/c of the "same" subtype, which is useful in maintenance and ordering of spare parts.
Of course, these are speculations - including a lot of "might-be's" ;-)

Concerning the "Industrieinstandsetzung" (roughly: factory repair) the story of the Australian War Memorial G-6, WNr. 163824(?), shows that even in December 1944 there were "old" parts (IIRC F-subassemblies) used to repair damaged a/c. In this case the airframe was even "downgraded" from a G-6/AS to a G-6 with a DB605A! So, "everything seems to be possible"...

As to the use of "old parts" in G-10-production, I had the impression, that the G-10 used tools and jigs from the Bf109G-production to get an a/c with nearly K-4-performance without having to retool the production line.
In this case subassemblies produced at an earlier date would be used if they fitted into the G-10 design.
As DIANA produced G-14/U4 of the 51x.xxx block before switching to G-10 (JaPo, page 10) it is possible that G-14 subassemblies like fuselages were used in the later production.

To return to my speculation, I don't think, that this usage would cause the fitting of a second data plate. As the a/c is finished at a later date and the main data plate already contains the "new" variant designation (G-10) it would not be useful or even confusing.

These are my thoughts - corrections, additions and further information most welcome!

Best regards

R.Zimmer