1963 letter rate from French Guyana to Malta

The cover in question bears the 1f Le Touquet issue and was sent from Cayenne on 6 July 1963 and arrived in Valetta (as indicated by the faint but legible backstamp) six days later on 12 July. Although there isn't as airmail label the dates confirm that this is how the item was transported.

At the time the basic foreign letter rate (20g 1.1.60- 17.1.65) was 50c and the European interior airmail fee (applicable here for the second leg from Paris to Valetta) was 20c. If these were the rates applied at the Cayenne office the item is overfranked by 30c. On the other hand, if the letter had weighed over 20 grams the postage would have been 80c and the air fee 40c resulting in the item being underpaid by 20c.

Either way, the rates do not reconcile with the 1 franc franking. Is it possible that there existed a specific airmail fee, applied at an office geographically outside Europe (although not politically so being an DOM) for mail between a DOM and destinations in Europe?

Any thoughts most welcome