I can tell you what happens in a 69 'S' which has the central oil tank, but I'm not sure about the fastback which has the panel oil tank, but I bet it's similar. The timed breather off the left side crank below the cylinders goes to the top hat thing on the oil tank. The 'S' central oil tank has a breather tube inside the top hat thingie, not attached to anything except the front of the oil tank and that goes out the front of the oil tank into the air filter area where I reckon the carbs are supposed to pull the oil tank vented vapors into the carbs to burn in the motor. Trouble is the vapor turns into oil and drips down on the air filter and then on to the gearbox once it builds up enough. It doesn't take much oil to make a mess, kind of like horse shit. That's why I put a catch bottle on it, and it seems to be effective, no more oil on the gearbox.
Now I've heard that some central oil tank people don't have this problem. Don't know why. I can say that most of what goes into my catch bottle is drips of water and over 1000 miles I have maybe 1/8" of oil in the bottle. It mostly happens on start up, but I've put a valve on the oil tank feed so I no longer wet sump, that may make a big difference. When it's wet sumped I'm sure lots of oil comes out of the timed breather and goes into the oil tank, where it may be partly vapor and is then passed on into the oil tank breather tube.
The early bikes also had a line off the return oil tank banjo that fed a chain oiler. I disconnected that as fast as I could and soldered it shut, it put a whole bunch of oil on the chain and it prevented a lot of rust, but a lot of oily dirt on the bottom of the bike too. As we speak, my centre stand is free of oil after about a year.
This is all my conjecture, I stand to be corrected, but I have a pretty oil free 69.
All things said, if oil comes out of that line, I'd put it in a catch bottle. If you drip it off the back, you're making a mess and it will get on the bike somehow from the wind. Mick Hemmings suggested I breathe the oil tank to the air, but I thought more of it.
Dave
69S