https://sweep-hand.org/want-an-original-citizen-bullhead-a-quick-buying-guide/

Citizen’s 8110A chronograph automatic high beat movement is a fine piece of engineering – compact in design with hand winding and ‘fly back’ capabilities it is a great addition to a collection. The model that it most often seen for sale – by a long way – is the Challenge Timer, now more usually called a ‘bullhead’. This model had a much longer production run than the other models in the 8110A line-up, and the ‘bullhead’ style is a popular design for today’s buyers.