This is Greiner #1a (first line of admonition on back panel is 78 mm wide / orange-red indicium) issued May 1, 1948.
The airletter was sent from DUISBURG-RUHRORT (cds 25.11.48) to Philadelphia, US via FRANKFURT (transit cds 16. 11. 48, on reverse).
On September 1, 1948 the postage rate for airletters was reduced to 60 Pfg. Post office stock of the 100 Pfg airletters was adjusted to the new postage rate in writing, as can be seen in the close-up below.
The message on the writing panel contains some interesting postal history information:
" [...] From now on we can send air mail letters and the first I send to the US is going to you. Again and again I have asked at the post office, if it still not possible to send air mail letters and they always told me, that this has not yet been allowed. Finally, it was allowed, but the postage could only be paid with International Reply Coupons, which the addressee in the US had to provide to the sender first. However, from now on we can buy these light airletters and can pay with German Marks [...]"