[RolandF]

Hallo John - my main concern is not the form 19-7-1940, which is a very uncommon way of writing a date but the combination of "Den" which under normal circumstances would require a location such as "Berlin, den 19. Juli 1940" or "Frankfurt, den 19.07.40" or whatsoever. "Den 19-7-1940" standing alone is definitely wrong , even not unofficial.
No German official would write the way it is done on the plaque.
John, which punctuation did you observe on the EKI in between the digits? 19 7 1940, 19-7-40 or 19/7/1940
Interesting fact is that the aces of spades emblem began to disappear from the airplanes shortly afterwards and was replaced by a red ring around the nose.