Yeah, it's a vent for the gearbox. There is supposed to be a rubber vent cap on the top of it as well, but 90% of ludes have lost these over the years.
Apparently it can be used to fill the gearbox up with oil as well, but I've never tried it myself
Ahh that's great to know. Last time I filled the gearbox up (around 5 months ago) the bottles had no spout on them so had to make a makeshift funnel/tube thing which leaked everywhere.
Only reason why I'm asking about it is my gearbox has lost a lot of oil and I can't figure out where it's leaking.
I have used the breather to put a little fluid in the gearbox, but only when I lost a little when I removed a shaft. Do not fill a gearbox up this way as you do not know when the gearbox is full.
I guess you could remove the proper fill up plug, then pour the oil down the breather pipe and stop once it starts pouring out the proper fill up hole (As you would do anyway). I guess it's just easier for some to do it from the breather.
I just use a hose long enough to thread down through the bay to reach the proper fill up hole.