Thank goodness for eBay.
The local craft store turned out to be no help at all for a 2mm bead to replace the missing ball bearing that somebody stupidly lost when stripping the paint from this machine. There's a hobby store nearby, and they sell parts for model cars and planes, both of which appear to use 2mm bearings in some elaborate mechanisms, but I didn't relish forking out a chunk of cash to buy one just to pull it apart, never mind trying to find a suitable replacement in the first place.
eBay came through, though, with a domestic seller "toolsupply" offering a bag of 100 2mm balls for a very reasonable price, plus free shipping. The parts arrived yesterday, and I set them aside after playing the usual round of Inappropriate Joke Time with my family ("Hey! Dad's balls are here!" "Honey, did you mail-order balls?" "Oh thank goodness: now I have lots of spare balls." etc.)
The replacement fits perfectly. And no wonder I couldn't find them locally: 2mm is a tiny little bearing, far to small for any application that I can think of, even for a bike, as recommended in the discussion in the last topic. It's seated exactly right in the opening, and the small tang that holds the ball in place now properly locks the return arm, as above.
Here's a shot of the inside of the return mechanism, just for completeness and archive purposes, in case I get a bright idea to take the arm off again and forget how it goes back together.
So, the arm is back in place at long last. I dropped in an old ribbon just to make sure I hadn't made any other horrible mistakes (I didn't) though I think I'm going to need to address that broken backspace sooner rather than later. It appears that the backspace is not only loose, but sometimes also hangs up the escapement which only a certain amount of jiggling can resolve. I might be getting back on eBay, checking out toolsupply's selection of springs, once I know what I'm looking for. Luckily, the backspace is accessible by removing the bottom plate alone, which is easy.
Naturally, I banished the crummy, stinky soundproofing foam from the bottom and side panels, and replaced it. This time I cut up a large piece of wool/nylon blend that I found in the remnant rack at eh craft store. It's held in place with automotive gasket silicon from the dollar store next door. The black looks pretty nice, what of it you can see.
I also lined some panels that weren't originally done, like the inside of the ribbon cover, and the inside of the panel that runs behind the paper table (behind the margins.) I also lined the inside of the knobs, since the paint bubbled up there. I like the effect of the red and black together, as you can see in the above pics. It's a repetition of the bicolor ribbon, and looks very typewriterish to me.
I did this mainly so I would not need to completely strip the inside of all the panels. I was pretty tired of the power tools by that point -- everything was done hand-held, since I don't have a proper workbench -- and more than once I wasn't paying attention and abraded some of the skin from my hand or arm with a tool. Ouch.
I think it was all worth it, though:
I'm still not convinced about the aesthetic quality of the plain silver. I did try affixing some art-tape racing stripes to the machine, but the tape was never meant to affix to bare metal, and peeled right off. The panels are steel, so magnets will stick to them. Perhaps I will go for magnetic stripes, or flames, or some other decoration. I'm not sure yet. But that's what makes projects like this so much fun: it's a blank canvas, and there's tons of room for customization.