You wrote ".0001-.0004" (1-4 ten thousandths of an inch), but 0.001-0.004" (thousandths) is more likely (sounds about right, I don't have my shop manual handy (I'll dig it up if someone doesn't volunteer it). The spacer is there to exert clamping force on the outer race of the bearing so that it doesn't spin in the hub (Tom Monaco told me he's repaired 'hundreds' of hubs). The possible problem--IIRC Steve G. has written extensively on this--is that some vendors sell a paper gasket that is so thick as to prevent the axle hub from sufficiently clamping the bearing. Some owners prefer to only use a sealant on the hub in lieu of a gasket, or to make a gasket out of thinner paper; that and the O-ring should prevent leaking (I use paper gaskets with a sealant). My guess, and it's only a guess, is that you want the gasket to be the same thickness, or just a 'smidge' thicker, depending on the gasket's composition/compressibility--than the distance the spacer ring stands proud. For example, if your spacer ring--Moss shows different part nos. for different cars--stands proud 0.002" then a gasket thickness of 0.002" would be perfect, but 0.003" might compress enough to allow proper clamping. I use Permatex #3 'Form-A-Gasket' in this type of application, as it works well and it's less likely to break off a chunk (I see no downside to using a sealant here).
I suspect this was an ongoing issue with Austin, or whoever made their live axles--MOWOG?--as the BN1s didn't have the spacer, or an O-ring, and the later cars have one for early 6-cyl cars and one for BN7s-BJ8s. The problem, besides spun bearings, is if you get a leak here it will soak your brake shoes.