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It is made of a different materal. Polymer Carbonate plastic... same stuff headlights and water bottles are made of. The others are made of Alkyd or Thermoplastic.
Either way. I've heard it looks cool for a while till you get a carbon coating inside. Then it looks dirty.
For a Holley Stealth Ram, it is mandatory. Me, I never cared for the coil sitting on top of the distributor. Too much engine vibration, IMO and a bit of heat. I put mine on the fender walls where it is cooler. Also, it allows me to have any coil I want. Somehow, I also like the idea of the coil apart so that it cannot act as a shield and trap heat. Maybe my imagination.
Brass is a mixture of copper and zinc anyway. Zinc is added to make it harder. I am gonna go with the red Taylor cap or similar. My car is red and I bought red wires so.......
In the past couple of years I've seen quite a few cap/rotor w/ aliumnum parts that cause running issues, most were less than 2 years old. Buy quality parts, you'll be happier.