You will find two versions in various widths in period articles and advertisements.
1) Hidden Lugs: There are five large bolts that bolt into the back of flanges in line with the spokes. I have talked with people that have had problems with the bolt holes wallowing out.
Well used Hidden Lugs Wheel
2) Exposed Lugs: There are five large bolts passing through the center hub between the spokes and fixed with washers and American Racing acorn style chrome plated lug nuts.
Here's images of a NOS 6X15 Exposed Lugs Wheel
Unlike conversion to six pin drive magnesium Halibrand wheels these American made wheels required zero modifications to the street car hubs, brakes, or steering arms.
Even though the 6X15 wheel shown is magnesium, the entire assembly is pretty heavy because the walls are thick in cross-section and they have all the steel parts bolted to them. On bathroom scales the 6X15 wheel shown is just barely lighter than the OE 6X15 Dunlop made wire wheel it replaces.