A simple envelope

During one of my visits to stamp exhibitions, I saw an envelope in a dealer's bin of mail that immediately caught my eye. An envelope covered with stamps and postage dues from the Dutch East Indies, featuring beautiful impressions of a Batavia date stamp. I bought the envelope and took a closer look at it at home. It turned out to be an envelope full of surprises! The envelope was a so-called postal stationery item with a value of 20 cents printed in blue and an image of Queen Wilhelmina in a crowned oval, but altered to 12½ cents via an overprint. These were available from April 15, 1930. The strange thing, however, is that this envelope without the 12½ cent overprint was never available! This can be recognized by the printing on the inside. The 'regular' envelopes were printed with type A and this envelope with type B. In addition, a pasted-on stamp, also of 20 cents and featuring an overprint, likewise of 12½ cents. The overprinted stamp was issued in 1930 and had a print run of 27 million copies. So, nothing special. From the Youth Care series of stamps issued starting December 1, 1930, a 5-cent copy was affixed. Only 172.182 copies of this stamp were sold. So, it is already a bit more special. As the fourth stamp, a 2½ cent numeral stamp, issued in 1922, was affixed. Millions of these must have been printed. The four values ​​added together totaled 32 cents. But is this correct? It could be, because an express slip was also affixed. The rate for a letter abroad was 12 cents, so the print on the envelope itself was sufficient. But the express fee was 25 cents, so 37 cents in total! So this was incorrect. The addressee was Mr. PW Broekman in Amsterdam. Seasoned collectors of postal items know the name of Mr. PW Broekman as the maker of many hundreds of 'special' envelopes and cards. But the address was crossed out, and written underneath was: per address MG de Bie, Batavia. So the envelope was NOT sent to Amsterdam (in which case there should surely have been an arrival stamp impression on it, and a departure stamp upon return from Amsterdam to Batavia). And FOUR identical impressions of the Batavia date stamp have been applied, with the date December 30, 1930. Stamped between 11 and 12 o'clock in the morning. But what are those two postage due stamps doing on the envelope? One of 2 cents, available since 1914, and one of 37 cents. The catalogue states that this latter stamp was issued in 1931! How is that possible? And why the 40 cents postage? The mystery deepens when the back of the envelope is examined. It bears two stamp impressions, one from the Government Main Warehouse Batavia Post, Telegraph & Telephone Service. Below that, MG de Bie….. Who solves the riddle of the postage due stamps on this envelope?