I can say on an American smogged 1973 FJ-40 the separator was present. It was mounted in the cab. The fourth line on gas engines led to the charcoal canister mounted on the passenger fender down below the battery. From there a computer controlled vacuum switching valve would allow engine vacuum to pull the vapors from the canister at certain times. These times, as I recall were during cruising down the highway and when the engine was warm. Other wise the fumes from the gas tank were trapped in the charcoal and did not vent to the atmosphere. The bottom of the charcoal canister had a breather hose to allow air into the tank as you displace liquid.
The three lines from the tank tap the tank at certain spots so the tank is always vented regardless of angle. The valve thing had to do with one way travel of the vapors and liquid not escaping as I recall.
What I don't know is if diesels can even use a charcoal canister. Or if diesels even need a charcoal canister. I suspect possibly that the separator was put on all cruisers after smogging was needed. But only the gas engines had a line connected to the forth line that ran to the charcoal canister. Otherwise, the fourth line is just dead ended for diesels. I would shape up a hard line, or find yours and run it up into the cab support like it was before. I think the object of this is to reduce dust and **** being sucked into the fuel tank.
Of course I'm speaking out of my ass, all I know is that my 73 had a separator and charcoal canister. It was the second or third year they did that. My FJ-60 also has a very similar system on it, aside from more controls of the charcoal canister cleansing, and the fact that the vapors from the float bowl is also directed to the canister, rather than being allowed to be vented to atmo.
Lots and lots of other rigs had charcoal canisters, but I have seen very few with the separator. I have the parts to put one back on my 73, finally, and will do so, as the expansion of the tank, especially inside the cab makes for a fumy ride and will push gas right out the filler it too full on a hot day. I made my own separator years ago, and it helped a lot. Used a piece of hard line made up into a coil to act as expansion room, then just vented the tank under the hood. This does make for a fire hazard though, glad I found the used parts to fix it.