This is one of the two vintage automatic chronograph movements made by Citizen, the other is the Citizen 8110A calibre that features an extra 12 hours counter.
A video of 8110A is here:
And here is the video for 67-9631, calibre 8100A. You can see the start-stop-reset mechanism in function, as well as the fly back. Also you can see the way you can change the date and the day. Enjoy!
More informations about this watch here: