I bought a set of Magnesuim 5-pin 427 S/C Cobra II rims direct from Halibrand in 2002 and was told they were the last set they had. They ran out of adapters so we used Trigo adapters and spinners. Pictures and details are at
Project 427 S/C.
The magnesium is a chore to keep up. They corrode if any moisture is near them. Polish back fairly good but a serious hassle.
Since bought track tires for these rims. For the street now use Trigo, chrome plated. There is a large difference in weight, about 20 lbs each as shown on the link above.
Not sure anyone will sell us real magnesium Cobra II rims these days. Let us know if a source is located.