Perhaps Wilier used both methods? When copper plating proved too costly, they moved to paint? The CC bike certainly has the look of oxidised copper, but none of the others on the steel-vintage site. The others are in remarkable condition, with minimal chipping only around the cable guides on the BB.
I believe the only way to settle this is to buy one for myself.
Yeah, big difference between the two finishes.
We'll eagerly await your addition to the thread.