The code h dating
You can partition the branch trees into several subsets that you know each subset has a different rate.
If some tip dates are missing, the program just takes use the subtree containing all defined date tips & nodes for the estimation.
The missing tip dates would be inferred at the end using the estimated rate & dates.
So razors made from 1930-49 can be difficult to date precisely.
However, blades made in those years were stamped with a date code: A for 1930, B for 1931, etc., plus 1-4 for the year quarter. In 1950 this system was expanded to include razors, too (see below).