Originally Posted by
Otis
As said it's for dripping oil into the head-tube. The version on the Frejus is not a zerk fitting for grease. Just spring loaded ball-check. You use a oil can with those. Grease would dry out. You could revitalize it with some oil. Same for hubs and bottom brackets.
This. It's an oiler. It would have been standard equipment on a lot of mechanical devices from that era. Quite often the spring ball would break and a screw would replace the device.