Vans VA-107 brake fluid reservoir. It's a cylinder with two bungs welded to it. Typical orientation is opposite of what you show in the picture. Nothing magical about the device -- it just holds a few ounces of your favorite MIL-H-5606 or whatever...
Here's a typical installation:
Normally the vent is at the top, inline with the major axis of the cylinder, and the output is at the 90° location at the "bottom". Nothing prevents you from flipping it around and changing the comes-into-from-the-goes-outta....
Remove the top plug & insert a dip stick. I have used everything from a narrow screw driver, piece of wire, sliver off a popsicle stick, what ever is handy. I fill the reservoir to about 75% full- don't fill it to the top!
