The outcome of the Administrative Conference held in The Hague in 1927 concerning air mail provisions was approved.
France presented a proposal for the establishment of mobile post offices. The first one was shown in 1934 at the Salon d'Automobile in Paris. The Netherlands followed in 1936 with an enormous truck, especially for sport events and tourism attractions.
The problem of underpaid postage on overseas mail was finally tackled at the 1929 Congress when it was decided that "no provision contained in the Convention prevents unpaid or underpaid items from being again presented to the sender for proper payment."
Also in 1929 model for the International Reply Coupon was changed. The 'London model' was available from 1 July 1930.
The small packet service (petit paquet) was created.
The British Post Office issued a series of stamps to commemorate the Postal Union Congress. To finance the Congress a one pound stamp was added to the series. This stamp was meant as a souvenir for the delegates and also to raise money. For the first time stamp collectors were being taken advantage of by the sale of high value stamps for which there was no obvious postal use.
Several colour trials were printed but in the end it was decided that the stamp would be printed in black. It is now one of the more expensive UK stamps in the catalogue. The colour trials are now part of the Royal Philatelic Collection.
Extra stamp catalogue - reproduced stamps