The question is how and what determents a model change or how long a production model lasts.

By numbers there were:
256,959 EH Models made between 1963 and 1965
178,927 HD Models made between 1965 and 1966
252,352 HR Models made between 1966 and 1968
199,039 HK Models made between 1968 and 1969
183,402 HT Models made between 1969 and 1970
155,787 HG Models made between 1970 and 1971
485,650 HQ Models made between 1971 and 1974
176,202 HJ Models made between 1974 and 1976
110,669 HX Models made between 1976 and 1977
154,155 HZ Models made between 1977 and 1980
60,231 WB Models made between 1980 and 1985