OK, MGB drivers, show me your Tom's Knob.
If you haven't heard of Tom's Knob you can be forgiven. Unless you've removed an engine or had water leak problems with the fresh air intake ahead of the windshield, you likely haven't seen it. If you have seen it, you may not know what it's called. Technically, it is a tube with a drain/dust valve or Moss Motors part number 363-170 (or number 56 in this reference drawing) but it has been known as Tom's Knob for nearly ever.
Its purpose is to allow drainage of water from the fresh air intake while preventing ingress of dirt, fumes and water thrown up from the road. It is found on the passenger side (in left-hand drive cars) in the transmission tunnel. You can see drain hole in the fresh air intake by looking down the fresh air box on the passenger side.
There's an excellent write up about MGB heater box drains here. Legend has it that the device was installed on the MGB and MGC (and GTs) bodies by a worker who's name was Tom. I can't tell you more than that, but I can tell you that Tom's Knob has a slightly, ah, phallic appearance.
Maybe all MGBs are male? Potty humor comments are welcome.
Photo Courtesy www.vord.net