Our 16 has this valve and a lot of pictures of other cars have them also. Dykes manual from 1920 has a good explanation
It may have had a priming valve mounted at one time, but that's not one. That's a drain type valve like for gas tank, radiator, or carb. Someone just used what was at hand to plug the hole.
These owner installed "air valves" were a popular addition back in the day. Many of the aluminum intake manifolds were drilled to accept an 1/8 NPT threaded petcock. The petcock was operated by twisting a rod running from the firewall to a hole in the valve handle. I had one on an unrestored 1910 I owned sometime back. It was sort of fun to use. When the car was running on flat ground, you would open it up and the car would hold rpms using less fuel. I always worried the engine would overheat, so I used it mainly to amuse/confuse passengers.
I've never understood the utility of these "air leak" devices. If you want to run a leaner mixture, why not adjust the mixture needle on the carb?
I use mine to shoot starter fluid in on VERY COLD mornings... Works great on a hand crank car. A lot cleaner than spraying sideways into the carb opening. My '15 has one, my '13 Speedster had one & our 1924 Studebaker has one as well.
My old '17 Indian Powerplus had a built-in air valve as described above. That was a Schebler carb standard feature on late teens Indians.
Notes from motoring pages of the day recommend the extra air valve as a braking device more so that leaning off the mixture device and warn of the danger of burning valves if using it so.