As there is some conjecture about how these double conical tapers were made, I made some enquiries on Practical Machinist website and was assured by a grinding expert that the sort of measurements shown would be entirely achievable in a production environment. So, the only remaining mystery is that of lapping/fitting the bearings. I believe they were had lapped, taper on taper. I estimate about 2 microns of material removed from the shallow taper would see the steep tapers to come into contact and lapping would continue on the whole bearing surface.
(I also show some Boley bearings being lapped for renovation)