Austarion

I have a conventional diff 3.545 out of a JA and the back plate has a hole in the top corner that i'm not sure what is meant to fill it with.. anyone know ?

Yep, that's the filler-hole.

From the factory, they had a rubber stopper with a little lip on it that just presses in.

I lost mine at some point too, so I just made something similar to the factory unit out of a cut-down bolt and some soft rubber hose.

There isn't much pressure inside a diff, so it doesn't need to be absolutely perfect. Provided it fits snuggly, it'll be fine.

Or, ask around on here, I'm sure there would be a few sitting around in people's garages!

The hole at the top of the back plate is for a rubber breather. If it has perished or dissappeared then you need to fit a breather/vent to prevent dust & dirt getting in. Fully sealing the hole is not recommended as it will increase likelyhood of leaks from other places (any pressure generated needs to go somewhere)

The oil filler hole is forward and below the axle seal in the housing itself. It's a threaded bolt similar to the drain. Fill the centre until oil comes out the hole.

is there something i can buy to let it breath or do i need to find someone who has one on here ?

cheers

i've never managed to remove one of these breathers without splitting it, so the easiest way to fix it is going to be to change the cover plate..