One of two of Citizen’s chronographs with case number only, i.e. no model number, this is a single register ‘Challenge Timer’ model which is driven by the 8100A movement (‘A’ is for Automatic, there’s no ‘B’ version 🙂 ).  Both the twin register 8110A movement and the 8100 were introduced in 1972. As with the 8110, this type has 23 jewels and uses the central sweep hand as the second counter, whilst the sub-dial counts minutes. Otherwise its features are the same, running at 28,800 beats per hour with hand winding and ‘fly-back’ (instant reset and restart of chronograph function without having to stop it first).

This model has a steel case with black coated alloy bezel. Two dial designs were produced as far as I know, the other having a silver sub-dial – the black sub-dial on this example is rather more subtle. The case is polished – I believe this is correct since two models shown in a 1974 catalog show one with a polished case, the other with a satin finish:

The dial and hands create a very legible face, and the highlights on the sub-dial give some colour and interest. The case back gives a production date of May 1972, so this is an early one:

Good examples of the 8100 are not easy to find, rather like some of the lesser known 8110 models. Unlike the 8110 models, I don’t recall seeing any made after 1974, so they had only a limited production run and fewer will have been sold as a result.

For more information on all Citizen’s vintage chronographs, here’s a link to my reference page: https://sweep-hand.org/citizens-vintage-chronographs/