The brass ring has diametrically opposite slots/notches for a reason. They are for a wide screwdriver?  That might answer your second question?

What it needs, to stop the leak, rather depends on where the oil is leaking from.

Edited By not done it yet on 31/05/2020 14:22:07

It would appear that the sight glass assembly is in two main parts, the chrome ring complete with glass and the brass fitting that merely pulls the fitting into/onto the outer face.

Remove the brass ring and the whole may come away. Clean, seal to outer face and retighten the retaining brass ring?

Do not have Senior mill, but that is what it looks like to me.

Howard beat me to it, how about a small section 'O' ring that fits in the chrome ring to seal the glass junction of the brass body at the end of the thread, if you get my drift.

George.

I think you'll find that the brass piece is externally threaded, and screws into the CI body of the gearbox. The 'slots' are there for a driver ( best made, with a pilot to fit the bore ). The chromed ring probably therefore screws onto an extension of the threads, trapping the glass between the two. Be careful when attacking the chromed part – it's likely to be brass, so easily marred.

Hylomar is a good sealant, if subsequent disassembly is required, but pretty much any low-strength oil-resisting sealant will do here. Loctite 567 Hydraulic Sealant (assuming the number is current) is a great sealant for drain plugs and any union which relies on the threads for sealing. It sets, but is easy to undo. I wouldn't use it on the chromed ring, however – that needs to be easily undone.