In the United Kingdom, a Silver Jubilee is held in a monarch's 25th year (followed by a Golden Jubilee after 50 years). Queen Elizabeth II's silver jubilee was in 1977.
... Kingdom) 1966 - Jackie Stewart[?], (United Kingdom) 1967 - Denny Hulme[?], (New Zealand) 1968 - Graham Hill, (United Kingdom) 1969 - Graham Hill, (United Kingdom) 1970 ...