There is a screw on the side of the base near the air flow..it's not a bottom fill, more like a side fill..
The screw is the air adjust. Next to it is what looks like a small hole, it has a check valve in it. You need a metal fine tip bottle to fill that way.
I fill my 3.1s from the top. Remove top, cover air inlet with thumb, fill to about half way up the threads.
Start threading the top on, turn upside down and remove your thumb from the air intake as you finish tightening the top. Take a couple draws to get the vacuum going. Done.