My thought is that it is decorative, an alternative to other colors that a writer might choose, similar to paints being available in different colors. Other usefulness might include beak's suggestion, and perhaps for filing of different topics the way NCR packets were supplied with different colored papers with copies to be given to customer, salesman, accounting, and inventory. Perhaps for enlivening the love letters of gigolos, where indeed an original would be in red ink too.

I note that Pete's Hermes Baby that he bases his opinion on is the later plastic-bodied version. While I agree with that opinion of that version, I also note that people who denigrate the Hermes Baby/Rocket almost never distinguish between the first metal-bodied versions and the later plastic ones nor even acknowledge the fact that that model was made over several decades, changing all the while. They are not the same typewriter in 1938 and 1965; engineers learned how to make things cheaper in order to meet competition. The model built its reputation on the earlier versions, which collectors encountered first, and rode the crest of the brand's exotic mystique (oh, those exotic Swiss; well, we were primed with the watch reputation and the conflation with French haute couture).

Can we get a comparative review from someone who has access to several examples of each era? The change in body material seems to have occurred in 1964. Were there major mechanical changes at the same time, in addition to running changes over the years?

This is similar to the more well-known distinctions between the Smith-Corona SkyRiters and the Smith-Corona Corsairs of the same time period, but in this case we have the advantage of distinct model names to trigger an awareness.
 

Great analysis of the question, SoucekFan! Got me thinkin'. (Wish this forum had an upvote system.)

Beachycove wrote:

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There is a centrifugal brake at the rear left that is part of the tabulator mechanism. It has a plastic gear (underlaid with metal) on which the plastic often seems to be cracked, and that can cause annoying problems like missing spaces. The trouble is that the gear effectively expands so that the teeth don’t mesh properly with the other gears present. Very common fault on the TM5, which you can read about online. Both mine were like this. Obviously this wa a weak point in the design. Best method of repair I have come across is to remove the brake (easily done), and stick a circular compression clamp like one of those little screw-on fuel line clamps around the cracked gear, in order to gently draw the plastics together (they should slide over the underlying metal). Then use an adhesive like shoo goo or epoxy to fix it for another fifty years. The machine will then likely outlive all of us.

This is exactly the fix I used for the same problem on the same machine and it worked great! Recommended.

zpark wrote:

Pete E. wrote:

Tolex is the wrap used on guitar cases and amps.
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I did not know that's what was used.  Did you find that you could have kept the hardware in place and strategically cut pieces of Tolex around it?  Or probably best to remove the hardware?

That's not what was used. Tolex was invented in the mid-'40s and is not known to have been used by typewriter companies. Even with musical companies, it was only used by some of them, though Fender was prominent. Beware of "Kleenex Syndrome".

But it is a good cover for our cases! Available from MojoTone, https://www.mojotone.com/cabinets/cabinet-parts-hardware/tolex-carpet-and-tweed?page=1

And here is a great source for authentic handle material and hardware: https://brettunsvillage.com/tools-hardware/trunk-parts/trunk-handles/

And here's one for feet: https://www.performance-pcs.com/search/?q=case%20feet

And here's one for everything: Note the categories at the left: https://www.mcmaster.com/