- I started building a database where various features are tracked by serial number and date of production. This data came from my observations and references, and as reported by contributors.
- A data table is provided for each grade in the monograph and on my web site. Please see paper for explanation.
- Note on this example AT&Co page, the change from sprung under (SU) to sprung over (SO) hairspring.
- Although this example feature change is rather quick, in general look for trends, not specific movements.