My 87 year old wheelbuilder friend, who has been building wheels for almost 70 years, tells me that they are built using normal spokes, but in all that time he only built up 2 pairs. He thinks the idea of the angled flange was to make them stiffer, but someone with a greater knowledge of engineering principles than me can probably work out if that is so. The only pair I have ever seen had a hairline crack through one rear flange. I think they were introduced in the 1950s.
Peter brown, Lincolnshire, England