Hereunder I tried to construct a timeline on the Ericofons history. Not sure if it is all complete but do not hesitate to send me any additional information or corrections.

Brief Ericofon Timeline
Era Year Event
1940 First design and patent investigations started in the ealry 1940's.
1950 1953 September 4th 1953 the first prototype was made.
1954 Serial production for institutions only.
1956 Serial production for the open market in USA Australia and Europe. 18 colors were offered in the USA through North Electric. Other countries offered 5 colors.
1958 Introduction of the Ericotone which increased popularity.
1960 1960 Design change to a smaller "new case" model.
1961 North Electric produces 100% for US market and18 colors was reduced to 8 colors.
1965 Introduction in Malaysia.
1967 TouchTones were introduced by North Electric in the US.
1968 Introduction in Singapore.
1970 1972 Production of Touchtones stopped (7000 produced) and North Electric sells Ericofon production in the U.S. to CEAC.
1976 700 goes into production in Europe celebrating Ericsson's 100th anniversary. It is assumed that 325 specials were made for the festivities.
1979 Introduction in The Netherlands with 3 colors.
1980 1982 Production of rotary models stopped. 2510000 produced
1984 Production of 700 model stopped. 30000 produced
2010 2013 Started my collection
2014 Birth of www.ericofon.nl

It is hard to tell when the earpiece was exactly changed on the new case shell. In my collection I have a flat earpiece from 1971 and a wedge earpiece from 1964. This would imply an overlap of at least 7 years.