I'd like to appeal to the knowledge of my artist and architecture friends, and pyat (whom I understand either works with office supplies, or with documentation for office supplies).
The main problem is that I don't know what lead size I'm looking for. I don't even know how old the pencil is - the range could easily be from the 1930s to the 1960s. I know that 0.9 mm is too small. At an art store, I tried a fancy 1.3 mm lead, and that seemed too big. (Although in retrospect I could've tried it differently, so that's still an option.) I've been told there's 1.0 mm leads, or used to be, but I can't find anyplace in Winnipeg that supplies them. I checked both the local art supply and architecture/drafting supply places.
So I'd like to ask my friends for a favour!
If you've got a really well-stocked art supply store near you, could you give them a phone call and ask them if they've got mechanical pencil refills between 0.9 and 1.3 mm? (There's a chance this pencil is Imperial, of course, which would make the closest unit 3/64", or about 1.19 mm.)
Here's what the pencil looks like. It's metal; my parents told me it's silver, but I have no idea if that's the case. I've got just a little bit of the original lead left. As for the mechanics of it, the black thing at the end can be twisted.
The only thing that holds the lead in is the tip, which has a very fine split in it to be able to pinch the lead. The ruler in this photo is not of high quality, plus this was taken at a slight angle, so the perspective may distort what the width seems to be.
As to how it works, the inside is made of two parts. There's a thin metal stick with a flat bit at one end with grooved edges. This is free to slide back and forth within the other part, the part that twists.
The inner shaft of the pencil is corkscrewed.
So when you turn the end of the pencil, the grooved bit is made to move downwards, and the thin metal stick pushes the lead out. Neat little thing!