[Dominik]

Yes, that is one of Lynn's (Trigo's) design.

Check their taper and if they are concentric. If not, improve it :-)

I would prefer a hex head (5/8"), if possible. It would not crack. A 1/2" thread in a 3/4" material would leave 1/8" of wall (?).

If I remember correctly, they once were $2 each. But $6 would be reasonable.
Anybody remembers wheels not coming off?

The taper is not needed, if the adapter is centered on the hub. However, most adapters come with a taper seat for the studs. Not the later ones from Vintage, though.

Dom