I find it better for something like this to use the real-world guidelines than model instructions
Eduard probably indicates 6 mm lines because Czechs don’t think in inches (and I don’t blame them).
As far as I know, the neatly painted aircraft are all later than the immediate invasion era. This makes sense, of course, since the planes were probably (re)painted neatly as soon as there was time. According to Wikipedia:
Which is to say, the day before the planned invasion. So it’s unlikely, IMHO, that many were neatly masked if whole squadrons had to be repainted in hours. I’ve long liked this photo for example:

As far as I know, the neatly painted aircraft are all later than the immediate invasion era. This makes sense, of course, since the planes were probably (re)painted neatly as soon as there was time. According to Wikipedia:
orders to paint the stripes were not issued to the troop carrier units until June 3 and to the fighter and bomber units until June 4.

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