I'm pretty sure that these are plastic - a sort of Bakelite.  I actually have an Underwood Five parts machine and I'm pretty sure the platen knobs are intact.  If you want one, I could check and let you know (it's in my loft !)

You have a lot of options, Shangas. If you are willing to carve one from wood (The Underwood 5 platen knobs were not made of wood.) then that is probably the quickest way for you to get a decent knob on your machine. Or you could use papier mâché and hold it on with a hose clamp. If you attach it by some means other than glue, then you can easily replace it with a better, more proper knob when one turns up.

The Underwood 5 is a very common typewriter, often avilable in "parts condition" where it would not be a shame to strip it for parts you need. Maybe not so common in Australia, I don't know, but a small knob can't be that costly to ship.

As to what the original material actually is: likely some kind of plastic (there are many possible kinds) or hard rubber or synthetic rubber. Probably not Bakelite, which is famous for being brittle, but maybe; it was in wide use in the '20s. (People tend to think that any object that looks old and is black or dark reddish-brown is Bakelite--it ain't necessarily so.) Then, too, this model was made for many years and the material specs might well have changed during its run. But in any case, you're not going to mold a replacement out of whatever we decide the material is, are you? If you can do that, you'll be in huge demand!

Good Luck and Have Fun!
==  Michael Höhne