Hi Tomislav,
That is correct...only some G-10's from DIANA had the second plate...
Originally I was concluding that the second plate was to indicate a "level of change" to the aircraft, done due to a bad test flight, or damage while still in the hands of the manufacture (while in transport, etc.). It would have been a good reference for the test pilot who sent the airplane back to manufacture for repair, to see a "change" indicated...knowing that the problems he pointed out were in-fact repaired and not overlooked.
Of course..checking my photo reference I have found unfinished airframes from DIANA that had that plate...so that probably rules out my theory...
Mike