The spring covers are plastic. But the original Boulder canti’s which the Onzas copied are all alum’ and better quality too (they don’t crack), but they’re quite rare.
Lots of boutique CNCd canti’s were made from the likes of Gravity Research, Machine Tech, Avid, Critical Racing, Joes, TNT, WTB, Crystal, Grafton etc, which used precision machined proprietary hardware. Not all were an improvement over Shimano though, performance wise!
My personal favourites are the Boulders, followed by Avid Tri Align, Paul Stoplights and Gravity Research Pipe Dreams.
The original version Campag Record OR take some beating though, they’re probably the most powerful canti’s I’ve ever used.