If this is a California model, there was a vapor recovery system; the fuel tank vent went to one input of a bottle-like device, which had something in it to adsorb the fuel vapors, and the vacuum line to the other input, to draw the fuel back out of the device and burn it. If not a CA model, the tank vent would probably go to an air filter box. I would guess that the four tubes on top of the carbs are bowl vents, and these normally go to the air filter box, so only clean air can enter the carbs that way, and any vapors released go into the intakes. I'm surprised that only one has a tube on the bottom, which I would guess is a drain; if there is a screw near it, it is very likely a drain.
This is all relatively-informed guessing; unless someone with first-hand knowledge of this bike chimes in, you really need a manual which shows routing for all those tubes.
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'68-7? CB450 and '98 Valkyrie Tourer, too