The 'reference' Underwood 5 cleaned up quite well too. It types still, although the 'a'-levers are a bit wonky and the line-lock needs fixing. (A replacement for the missing set-screw for the line-lock was found in the special typewriter-bits tin - adjustment is an adventure that will be tackled later.)
It is a 1928 machine, but looks a bit anachronistic with its more modern ruler and the large, plastic platen knobs. The carriage-rack is also a later replacement. It means it has a nice corrosion-free lever, but the rack just fouls the backspace-mechanism when pressed down (carriage release). When moving the carriage with carriage-release pressed down it goes into a high-pitched squeak (should drive dogs wild). Despite all that, it types and is very much an Underwood 5 Standard Typewriter.
Compared to the 1920 Underwood 5, there are a myriad little changes. From the ribbon-mechanism contrates now being spoked, to an entirely different frame casting. Partly because of different mechanisms for the ribbon and for the butterfly-scales, partly the differences seem stylistic only. The pillars at the corners of the frame no longer have clear 'capitals' and the bevels at the front of the frame are also slightly different. Even the carriage casting is different, the right return 'hook' has a different form from the 1920 machine.
One justification for getting the 1928 machine was its cork platen. The rubber on the 1920 machine was rock-hard and liable to damage the type-slugs. As I'm not sure yet if I can get cylinders re-surfaced with rubber locally, a cork platen is a great alternative. It may not be as 'grippy' as a new rubber platen, but it will keep its resilience and should last near-indefinitely.
So removed the cork platen out of the 1928 machine. As bonus, it has a matching set of cork feed rollers. Four rear and four front feed rollers:
With all parts exchanged, the shiny 1920 Underwood 5 typewriter now has a cork platen and rollers. This is much healthier for the type-slugs and the quality of printing also is clearly improved.
Apart from having had to swap paper-trays too, this is all as was the plan.
(However. Letting the '28 machine sit with the rock-hard platen now seems 'off' and a pity. To scout for a way to re-surface a rubber platen locally...)