Runner gearbox leaking from breather hole

I am baffled by this, maybe someone can help...
This is a 172 build in 125 cases with a 180 gearbox leaking oil from the breather. It's obvious how it's doing it but why would they design it to do that and what can i do about it?
Red circles are slots that are cut out
TIA

 

Reference to image 1:
I don't think there is supposed to be a slot cut out of that very top web section.
I found this, it might help if you blow the photos up some:
http://www.shiny-red.net/guides/gearboxes/

If it's not supposed to be there, build it up with chemical metal to seal it.

 

Thanks Steve. My mistake. After cleaning it up, turns out the slot isn't there on the case (image 1) - as you say. However, the slot is there on the (180) backplate which means it just goes straight through the slot and out the breather (which are in the same place, also in the red circle).

That link shows a case different to mine, it looks like a large drum/ disc case. Mine is a small drum. I can't see the breather hole either because of the angle of the camera or because it's not in the same place.

I remember working on one of those and the breather (if i'm not mistaken) is higher up and away from the slot - that would work.

I can't get my hands on a 125 back plate. I'm curious if that slot is there on one of those?

If it isn't then i simply need a 125 backplate for the 125 case and remove the blockage circled green - that would work.

 

There's a recess (circled) on that case, that mine is without; I think that must be for the breather.

 

Sorted