The Federal requirements were funny back then, as the side marker lights began to appear on "American" cars in '68. As for most British cars, they only got side reflectors for the 1969 model year and actual lamps (as well as reflectors in most instances) for the 1970 model year. This was NOT a requirement in most other markets, though, especially the UK, and it so often is a home market car that one sees photographed in books about cars.
But yes, any number of folk have deleted the side marker units over the years...for whatever reason(s)!