That's a 550-four. The Honda CB500-four ran through the 1973 model. Seems like Honda might have produced a 500-twin... pretty sure they had a 500-single in the late 70s. Both the v-twin and single Ascots might have been 500s. A lot of Japanese bikes in the late 70s were 699cc because of the import tariff on 700cc bikes that saved Harley-Davidson from shutting down.
Edit: the Harley 700cc tariff was passed in 1983 and the 500cc Honda Ascots (v-twins and singles) were 1982-1984 models. Honda also had the 500cc V-4 in the early 1980s.