What is this valve and how do I fill it?
This is a Dunlop valve, also known as Woods. These valves always let air in but not out. The nut holds the valve in place, and you are supposed to unscrew it only if you want to deflate the tube.

There are adapters from Dunlop to Schrader, but the most common way to inflate these is a simple hand pump that has just a rubber gasket instead of chuck like shown in your picture.