Belgium
Short Paid Aerogramme

 

Belgian 4F aerogramme (Greiner no. 14, watermark I) issued Nov 7, 1958.

Used from BRUXELLES (cds 13-5-68) to Vienna, Austria. 4F was the (state-subsidized) rate for aerogrammes to the colony of Belgian Congo; the rate to European countries was 6F. Hence, the aerogramme was short paid by 2F.

The Belgian taxation "4/6" shows double the deficient postage (2 × 2F) in the numerator and the first weight step rate for surface letters (6F) in the denominator.

Based on the first weight step rate for surface letters from Austria to Belgium, which was at 3.50S from Jan 1, 1967, to Jun 30, 1971, the postage due calculated in Austria was: 

4F / 6F × 3.50S = 2.33S and rounded up to the nearest 0.10S, which gives 2.40S as in the Austrian postage due mark (“Nachgebühr”).

 

References

Greiner refers to  Greiner/Greiner, Aerogramme von Europa, 3e, 1993