Early 1950's - the years in which Europe dug out of World War II and the Western European countires were able to develop an aeromodeling hobby. The first truly international speed contests for the 2.5 cc class was, to my knowledge, in 1951 and 1952. Here are some articles written during 1952-1955 that describe those first competitions. At this time the 5cc and 10cc events were more popular.
1952 - The CIAM meeting. Why there was no 2.5cc Championship in 1953 and other matters.
An Obituary for a Pioneer in 2.5cc speed Peter Wright