Citizen’s best known vintage mechanical chronograph is usually called the ‘bullhead’, a name that is of course not specific to the Citizen brand. The correct original name for Citizen’s bullheads was ‘Challenge Timer’, but only recently I have found that this name in fact applied to all their chronographs. Here’s a scan from October 1972 marketing material, showing two of the 8100A single register chronographs (NB: I think the blue cast is caused by the printing):
1972 was the year Citizen launched their chronograph watches – so this image was part of the regular newsletter to dealers which presented, among other things, the new models they would be selling. I believe the 8100A was launched first, with the 8110A following in 1973 (gleaned from Citizen’s own historical information).
Here are the same two models in the 1972 catalog, with retail prices (about JPY5000 less than the 8110A two register models) – these images are also good for showing the original bracelets these models came on as well as the type of case finish (NB: note dials both look to be black in this image, I’ve not seen any examples of this model with a blue dial):
These two are variants of the same model, which had no ’67-xxxx’ model number, only a case number – 4-900014 – so these may have been the first of the range.
My own example is from May 1972 – it was made that month, but these were first sold, according to Citizen, in October of that year (which fits with the date of the marketing material):
The back gives the early production date: