Citizen’s mechanical chronographs from the 1970s are relatively well known, especially the ‘bullheads’ with the two sub-register 8110A movement. Known at the time as ‘Challenge Timers’ Citizen’s range of models also included single sub-register models – in fact these were the first to appear for sale, in October 1972. The 8110’s were launched a few months later, in February 1973.

These single register models use the 8100A movement, and examples of these are generally speaking harder to find than the 8110 versions. They feature a minutes sub-dial, with a full size second hand, but no hours sub-register. The 67-9631 is one such model:

The wide ‘cushion’ style case with no bezel is unusual, and the dial is perhaps at its best out of the case:

The movement has the same specifications as the 8110 – it is high beat, running at 28,800 beats per hour and with the fly-back function:

The serial number gives a production date of April 1974:

The 8100s had only a short production run – I don’t think I’ve seen one later than 1974 – which of course helps to account for their scarcity these days.

Finally, this one is now on an original bracelet: