The situation here is: the 750 came with 2 different types of oil tank vent systems. Benjie only makes one type of oil tank...the simple one.
The 1969-1975 CB750K (aka K0-K5) has a vented oil cap and a vent tube between the oil tank and the rear of the engine: this port has a separator/filter/screen inside the tank (the connection is hose #7 in the CB750K1 Oil Tank" pic below). It also has a road draft tube from the top of the engine's cam cover that goes down to the back of the engine and, depending on the "K" model, either to the right or left side of the swingarm pivot where it can vent the cases below the bike. It's usually best if you can direct this tube into the chain drive area, to take advantage of putting the oil vapors to work on your chain.
During the K6/F0 era, it was changed to have a sealed oil tank, using a nylatron oil dipstick that screws into the tank. This is accompanied by a complex filter-separator-vent chamber under the battery that accepts hoses from the back of the engine and oil tank, and the airbox. (See the "750 Separator" pic below.) The general idea was to try to capture the "noxious crankcase fumes" and send them back to the engine to be burned. Mostly, these plugged up to a non-functional level by about 8k miles, and no one ever knew the difference. After that, the crankcase pressures rose, helping the engine to leak (weep) in the process, which is why so many of the post-1976 engines end up with weepy oil leaks all over. The pressures slowly force the oil against the gaskets until it eventually dissolves the goo that is impregnated in the gasket material, letting the vapors leak out as oil weeps here and there.
When rebuilding these bikes, I usually install a road draft tube wye or tee joint above and behind the airbox, and join it with the one from the back of the crankcase upward to the one from the top of the engine, connecting together up over the back of the airbox somewhere. This keeps the oil mist from the back of the engine from turning into a liquid form by letting it drain back down into the back of the engine. The open end of this joined hose then goes back down into the countershaft sprocket area to help lube the chain a tiny bit more.
For the most part, replacing the old fume filter parts in the separator assembly is either expensive, impractical, or impossible today, and cleaning them doesn't work very well once they have plugged up. Interestingly, using lighter oils (10w-something weight) will let this gizmo last a little longer (maybe 15k miles) before it plugs up, but it still plugs up.
In the attached pic, the "mystery hose" that is shown from the separator with no part ID is the one that goes to the oil tank. The #24 hose goes to the back of the engine. In the earlier bikes, the #24 hose goes directly to the oil tank vent, where the tank has a foam (or screen) separator inside. If Benjie's tank does not have this hose port, I suggest making one? It has to sit above the "full" oil level mark on the dipstick, and point downward, at the least, to try to prevent sloshing oil from working its way into the tube and going back to the engine (it's supposed to be flowing the other direction!).
I like to advise removing them and throwing them away because I don't like the EPA nor it's irrational, uneducated, vendetta-oriented edicts, and this is my revenge...but that's a topic for a different forum. Honda was the ONLY bike maker targeted to make these types of "improvements" in the mid-1970s, because of political [crap] pressure from Washington in those days. Kawi, Suzuki, Harley, and Yamaha were exempted until the late 1980s. That's called 'tyranny'....
