Where it all began

The first postcards produced specifically to accompany an Expedition had been issued in 1901 for the German South Polar Expedition.
In the UK, this idea was exploited in the same year by E. Wrench of London, who produced four "Links of Empire" cards for the British National Antarctic Expedition. One of these (infamously featuring a Polar Bear!) was advertised as being "carried on the sturdy vessel amongst the icebergs and floes of the vast unknown of the Antarctic Seas. It will be posted at the earliest possible port that boasts a post office when the explorers return".
The four cards could be purchased in advance of the Expedition's departure, for 2 shillings (£0.10), and were resoundingly popular.

The idea of the "subscriber" card had been born.

Pride of place as the originator of Himalayan Expedition postal history 'collectables' must go to Captain John Noel. Captain Noel had been instrumental in stimulating the Royal Geographical Society to proceed with the idea of an attempt to climb Everest. Noel effectively underwrote most of the costs of the third British Mount Everest Expedition in 1924, by purchasing the film rights. He also conceived and designed (with the assistance of Francis Helps) the famous Blue Label and subscriber Postcard for this Expedition as a way to promote the forthcoming film and recoup his investment.

From the News Chronicle, London, 13th February 1924.