The upper item is among a group of covers which I bought cheaply at a recent auction. It was a very pleasant surprise as it was not listed in the catalogue description. It is uncommon rather than rare. A similar prewar item is very much on my wish list.
So this was an incoming philatelic item which was underpaid and received a T handstamp and inscribed 6 centimes in old French money as per UPU regulations. This translated to 5c in local money. This was paid before the cover was released to the recipient. Two stamps for a total of 5c was fixed to the back and cancelled. There were no official postage due adhesives postwar. In the previous era, the postage due adhesives were fixed and cancelled on a separate memo. These were flimsy pieces of paper. Not a lot survived and they are definitely expensive. From the dates one could see it took a total of 11 days for the cover to be delivered which is probably faster than nowadays.
This is not a postage due cover. There are quite a few of this type of covers around. They are attractive and there was a variety of adhesives or combinations used making each item slightly different. This cover was carried by one of the two Short "Singapore III" flying boats of No. 205 Squadron under command of Wing Commander T.W.Scott, returning to Singapore from where they had departed on June 22nd.