I, being close to the source, see them pop up regularly and they do not command high prices.
Neither did this one! lol
Thanks, Johan. I collected it for the images of the Lockheed Electra (aircraft on covers) and for the question marks airmail border (variant airmail borders), thus an interesting cover for me. I always have been curious as to what made the flight mysterious.
25,000 is, indeed, a bunch of covers. KLM must have given them away to anyone who would take one!