This is an Underwood Standard I picked up at an antique store. Its serial number is 4348108-14, which according to this page dates it to around 1935. It was in pretty decent condition and seems to work pretty well.
(lame perspectiveish drawing)
It's broken in half just left of the screw-hole mounts which is why it looks crooked in that picture. It's about 4.6cm long, and about 0.4cm wide.
If you take a look at the mechanism, here's where it goes (I've taken the top cover off of the assembly and unhooked the spring, clearly... You can click on those to get a slightly bigger photo of each) -
Luckily, it's not a terribly vital bit... It looks like it's primarily so that the whole mechanism doesn't slide out too far (so it's not pressing against the carriage return lever constantly). It's also probably one of the easier bits to just fabricate if a replacement can't be found. However, anyone know the best place to find an actual replacement?
I think that perhaps it's there to take some of the impact from the keys? I'm not sure exactly, but mine's about 90% missing. Only the extreme corners remain. It's of this sort of crumbly substance. I'm not sure if I should be concerned with finding a replacement for that, as well.