Very unusual on a standard production rear axle. The hole isn't hidden by an accumulation of grime is it?
I would be inclined to fit a breather - in the rear cover if necessary.
From Morganatica:
Routine servicing
Oils should be drained and replaced every 5,000 miles. An extreme pressure SAE 90 hypoid [GL5 in current specifications - PJB] lubricant should be used. Do not mix oils. There is a vent hole about four inches to the right of the junction of the right hand axle tube and the centre banjo. This could be on the top or at the side. Make sure this hasn't been sealed over with paint. NOTE Quality control at the Salisbury factory was problematical and on some assemblies this breather hole may have been overlooked. On one axle it was found on the underside, not a good idea!
