I have found out that for the upper vent VW T3 bus parts fit with the part number
251201311B and
N90100501 for the grommet. Advantage over my 74 original one is that it has a check ball, that will prevent fuel running trough. I think float valve is the right wording.
But hopefully you find the one you have. It would be interesting to me, if yours has also a float valve or not.
I think on this forum was a very nice build of a orange t-top with a lot of pics. If I remember correctly there where some pictures how the solve that. I'm sure
@LILCBRA knows the build.