Gone for a single sub-register chronograph today, i.e. with the 8100A 23 jewel movement. This is a 67-9551, with black coated case and a stainless steel bezel – so the case material code is BLS. This one is from March 1973, and is on an original resin strap. I’m not sure if this was the strap for this model though, so please let me know if you have seen an original catalogue scan or advert. The dial design is similar to the more familiar 67-9119 (8110A) model although it does not have the inner rotating ring featured on that one. I’ve not seen very many examples of the 67-9551 over the years, so it’s a nice one to find if you can:

Here’s a pic of the clasp – this type of strap does split over time, so finding on in good order is difficult:

SONY DSC

More info on all of Citizen’s automatic chronographs here: https://sweep-hand.org/citizens-vintage-chronographs/