I have seen similar hub problems on 1950s steel "British Hubs" fitted to the back wheels of Dot and Greeves competition machines,where the sprocket bolts were forever coming loose.The previous owner no doubt had found that instead of the wheel nuts coming loose ,the action of the wheels over rough ground rattled the short end of the studs out of the hubs,the wheel nuts did not come loose .to cure the problem he reversed the studs and put a lock nut on the back of the hub, My solution would be to remove the hubs and drill and tap 3 new holes eqispaced between the old ones,as the back of the hub is possibly as cast ,a spotface should also machined around the hole, find some longer studs so that they can be screwed into the new holes and a nut to lock the stud fitted at the back of the hub,the spot face is essential to give a flat seating for the nut,its preferable that the nut is a Nyloc type nut, By drilling new holes you get a good thread,and you have a wider choice when looking for studs as they can be any thread and the holes tapped to suit, it does not matter if the thread is finer than unc, metric or BSF would be ok. Its just that the stud must remain really secure in the hub. I cured my Greeves hub by machining some 5/8 dia steel 1/2 inch long,brazing them in line with the worn threaded bolt holes in the flange then brazed them onto the inside of the spoke flange, It was quite difficult keeping the oxy torch away from the spokes i then used the sprocket as jig to drill through the brazed on bushes,then tapped the bushes and then had a long thread to secure the sprocket bolts, In more recent times I restored both a DOT and a Greeves trials bike though this time it was easier as I did it with bare hubs before I respoked the wheels. Happy days.