99 deluxe oil tank plumbing

Thats a modified tower.  I would logically expect the lower pipe to accept the breather from the crankcase  (which is a modification as the std engine pipe just went to the ground or was often bent up a bit to lube the chain and tyre!) . The  non std upper pipe  with screwed connection I would expect to lead to a hole in the top of the rear chainguard  near the brake switch.   The orriginal tower connection for this appears to have been cut off and blanked. This layout is better than the std settup which often  left puddles , but  if used for short rides the slightly accidic  moist fumes can rust the chain. Perhaps better to cable tie this  to exhaust to the right side of the bike below the swing arm , and your sins can be hidden behind the side panel . I used to remove and wash my chain once a year in petrol ,soak in hot linklyfe  then regularly brush on some EP 90 . An extra hole in the bottom rear of the enclosed case allows  mess to drain away Now I have a Scottoiler which would be ideal if it did what I tell it !!.

The DeLuxe tank tower was just an extension of the filler in order to make a top up more likely to go inside the tank rather than down the outside.  The non-DeLuxe slimline tanks from 1962 on had a sealed froth tower which trapped the fast moving breather gases, condensed the vapour and allowed it to trickle back into the oil tank.

The attachments show what the connectors should look like.  As Robert says, Peter's tank looks like it has been modified for a higher volume flow, probably from the engine breather.

Studying your photo  there may be more going on.  Do you have 3 connections for pipes?. Maybe 2 breathers in and one out.