This was left inside my copy of One Man's Muddle by E. Baker Quinn, a 1938 reprint of a book originally published in 1936. The card itself is undated so I can only guess it comes from no earlier than 1938 and probably no later than 1940 prior to the Blitz. I can't imagine there would be too many solicitations for building societies offering such a high interest rate after London was bombed.
Building societies seem to be peculiar to Europe. From what I gather they are analogous to credit unions in the United States as they are owned by all its members. They also sound like a savings and loan institution which used to be prevalent in the US prior to the scandals that rocked the financial world back in the 1990s. All three types of financial organizations used the members money to make mortgage, car and other personal loans to its membership.
The British building societies date back to the late 19th century. On this Pall Mall Building Society solicitation card they remind you of that with the statement: "Incorporated under the Building Societies Ats, 1874 to 1894." Pall Mall apparently did not have a long life. I can find nothing about the group on the various historical financial websites I visited. A Wikipedia article on building societies only lists current histories dating back to the 1990s and does not track anything that went out of business or was acquired by another banking institution prior to 1944.
The Honorable Ralph Roper-Curzon (1899-1979), the Secretary of the building society to whom the card is addressed, was the second son of Henry John Philip Sidney Roper-Curzon (1837-1936), the 18th Baron Teynman. Ralph served in both World Wars, was wounded during WW1 and achieved the rank of Captain and Acting Lieutenant-Colonel in the Scots Guards. Two photographs of Ralph are part of the extensive collection in the National Portrait Gallery but they have not bothered to digitize either. Darn! However, his father and older brother who were both listed in Burke's Peerage had their photos taken by Bassano Ltd have been digitized and can be viewed online. You can see Henry here, and Christopher John (1896-1972), his brother, here.