Wiegand's (2000) "Katalog der Aerogramme von Asien" states that those two aerogrammes were never sold at post offices and were not valid for posting. Wiegand writes that the aerogrammes were only sold with an overprint OR with a stamp affixed that covers the indicium.
According to Wiegand, the version with the overprint is scarcer than the one with the stamp covering the indicium.
I came across a copy that has both the overprint AND the stamp covering the indicium.
This is how the overprint looks like (most likely, the stamp has been removed from this one):
And this is the aerogramme which has both the overprint and the stamp covering the indicium.
Close-up of the indicium area as seen from the aerogramme's writing panel.