QSL Museum

The concept of sending a post card to verify reception of a station (and later two-way contact between them) may have been independently invented several times. The earliest reference seems to be a card sent in 1916 from 8VX in Buffalo, New York to 3TQ in Philadelphia, Pennsylvania (in those days ITU prefixes were not used). The standardized card with callsign, frequency, date, etc. may have been developed in 1919 by C.D. Hoffman, 8UX, in Akron, Ohio. In Europe, W.E.F. "Bill" Corsham, 2UV, first used a QSL when operating from Harlesden, England in 1922.

From WIkipedia