Spurred on by extra datapoints of Underwood typewriters left-foot numbers on The Database (Thank you James!), and now looking only at the Underwood 5 samples, there are enough points to reasonably map a relation.
It is not quite a linear fit (the portfolio mix of Underwood was evolving), but a polynomial fit gives a pretty good correlation. Using that fitted relation and the serial numbers from the agelist, then an 'auxiliary' agelist based on frame numbers can be created:
This agelist from frame numbers (a.k.a. the front-left foot numbers) probably has a margin of about a quarter-year either way. Note that after 1926 the first digit of the frame number can be missing, but the full number likely still stamped on the carriage side.
This 'auxiliary' agelist very likely can also give a year-estimate for Underwood typewriters that fall in other serial number ranges than the 5, but were built from the same frame-casting and assembly line.
There's more information still to be had from more data - e.g. there are hints that 1916 saw a significant, sudden shift in the Underwood portfolio -or factory set-up. Also there are hints on batch-sizes, and production buffering was not completely first-in first-out either.
I.e. as more frame-numbers become known, the data will improve and more information can be extracted.
But already possible is a rough "front-left foot agelist" :-)
Great work. With more WUS info, it'll provide some date info there. Need those late 5s near the ribbon switch over for the end of the run number. That will provide a closest yet approximation of how many 3/4/5s were made. Then add the ribbon switch update machines from 370 to 399 for a grand total. And this be the end of the "it is a rebuild number" 'theory'.
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