What is that?!

I think the OP is right, its a accelerator pump breather/weep hole. I had one on a 1920 that actually spewed fuel out when the pump seal went bad. It was unreal, a jet of fuel just streaming onto the intake/exhust manifold. I put a pic of it somewhere on this site 5-6 (8!) years ago. IIRC, mine was behind the bowl casting too. Found a great post with a pic and explanation of its purpose.
Holley 1920 carburetor is leaking gas on my 72 /6 duster - Slant Six Forum