Oil pump in/out is at the back, so the top fitting could be a breather. However it's possible that the output of the oil pump goes up to a sight feed on the tank-top, then down to your fitting to go into the motor. Common in the mid-late 1920s.
Check to see if there is a flap/ball valve in the fitting, and if so whether it allows flow in or out.
If you know what bike the motor comes from, you should be able to establish how the lubrication system works.
Leon