4-synch gearbox - OD and non-OD are the same:

3-synch non-OD: (Chicagoland MG Club)

However despite one being listed in the Parts Catalogue there are several indications in the wild that 3-synch OD don't have one, just a hole in the side of the remote change unit:

Then again this does seem to have something fitted in that hole (yellow arrow, ignore red circles):

Cap removed. The cap seals the end of the tube, and the holes in the sides (one each side) allow for the breathing. Martin Roberts pointed out that they can be used with a small screwdriver or thin bar through the holes to fit it - or to remove one that hasn't already broken off. The cap should shroud the holes to hopefully prevent flood water going in, but it's a bit hit and miss. (Dave O'Neil on MG-Cars.net). I suspect this is the earlier breather as a new 21H6060 in 2017 is quite different:

A new 12H6060/V8 breather, comprising a body (A), insert (B), and cap (C). It can be seen that the main part of the body and hence the cap are significantly bigger than the original items above:

Whilst the body has indentations (arrowed), they are not pierced as in the earlier breather. Together with the cut-outs in the ring around the body (which the cap snaps onto), they allow air to pass between the cap and the body:

The insert. The small hole visible in the first image is actually too small to allow water that might be sitting in the insert to run through to the axle. Of course, if the axle is hot, and has run through water, any sitting there will be sucked in as the axle cools. The slots across the top allow air to pass between the insert and the cap:

The insert fitted to the body ...

... and the cap fitted: