In theory, any oil should EXIT by the hole(s) in the bottom along with unwanted moisture, rather than get sucked in through it/them - but anything is possible I suppose.
However, gasket failure = leak to outside world rather than straight into mag; oil seal failure = leak into mag and only to outside world when the mag gets filled up! So the former isn't so likely I don't think if the mag's all slimey.
The chamfer point Julian makes shouldn't affect the fit of the flange joint at the rear of the timing chest, but it makes locating the long 'nut' easier on the engine side low down. I've run my A with several mags over the years, with and without a chamfered bottom edge, and there's no prob with the fit at the flange (swing-arm bike) - not sure if things are tighter on bolt-up 'box versions though. About 65% of mags I see off As are 'correct', the rest are 'whatever', given that all the basics are the same.
If the oil seal has died, there'll be oil inside. Probably won't stop the thing working, but not good all the same. The main earth brush will be a very oily indicator of trouble, being that end, but the cb area may stay dry or dryish for ages. A slightly awkward fix as it requires extraction of the drive end bearing's outer race to replace, which can be a swine to get out sometimes.
