FWIW you can buy bags of grade 10 stainless bearings for not much, last shop i used to work in bought them at about £15 per 100. As opposed to £8-10 for grade 25 and even less for "bearings" from the local wholesaler.
The grade 10s were usually in a named grade of stainless, with a stated hardness as well, grade 25 and "bearings" weren't, just "stainless".
On the plus side, it was £15 extra on a wheel service to have grade 10s fitted, and same again to have them in the bottom bracket as well. (now that shows how long ago it was.)
A quick google shows you can currently get them at about £20 per 100 in 1/4" stainless.
Servicing Campag Record OS silver hubs (pre-2006)
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It looks to me like they would be 5/32 front and rear but I'd wait for someone more expert to reply. Also, you cannot, I don't think, buy the cups and cones separately so you may be better off just buying the kit. That seems to be the case for newer record hubs anyway but maybe not for your generation of hub.
Sorry, that's a lot of maybes...
EDIT...ignore the kit part above, found a couple of places where you can buy the separate components.
Sorry, that's a lot of maybes...
EDIT...ignore the kit part above, found a couple of places where you can buy the separate components.