Or, as it's otherwise known, the brake and clutch fluid reservoir. Spent a while cleaning it up today, to see if it is worth restoring - and I think it is. Not in perfect shape, but as long as it doesn't leak it should come out ok. Most of the discolouration is just light corrosion and surface rust. The nice thing about using this one is that it's original to the vehicle. Just have to find a way to make sure the bottom exit doesn't leak.
In this view the basic components:
Inside is nice and clean (except some dust from the cleaning!).