Contrary to the genuine intention to not get a reference machine, I did get another Underwood 5 typewriter as a 'reference machine'. Had not meant to get another one of these magnificent 'desktop-cathedrals', but it was local, not too expensive - plus it had a cork platen and some good nickel to perhaps replace a few badly rusted parts on the 'good' 1920 machine. So; one more Underwood Standard Typewriter:
Going by the serial number, this is a 1928 machine. It has however been serviced and 'upgraded' over its working life, up to at least the 1950-ies. It does not have its original ruler (but a later style of Underwood ruler) and it got larger, plastic platen knobs (not Underwood items I suspect).
This typewriter was not protected by a case, but it did come with a dust-cover - probably also a 1950s article. Despite having this dust cover, there is some dust...
Luckily it is very 'loose' and fluffy dust. Have been cleaning it up and exploring ("What comes off when I undo this screw?"), starting to learn how to take these apart (and put them back together!). Thankfully there are many excellent resources available on these machines. Especially the postings on the Underwood 5 by Mary E at MyOldTypewriter were very helpful with lots of clarifying pictures. (Thank you!)
Am however starting to see that the original plan to use the cork platen+rollers and some of the nickel-bits is hitting a snag. Those Underwood 5 typewriters may look pretty much alike, but... ...they are surprisingly different!