Steel, zinc, or aluminum plate of ‘The Tragedy’?

My first and only though is that I never heard of steel being used as a source of plating.

My thought, which I admit is a little off the wall, is that plates is Cockney rhyming slang for feet, plates of meat.

Something I read about Picasso’s “Saltimbanque Suite” at Christies.com, regarding Ambroise Vollard’s purchase of them in 1914. (https://www.christies.com/features/Picasso-prints-guide-10144-1.aspx)

“The works went largely unnoticed, though, until the art dealer Ambroise Vollard set eyes on them six years later. Having bought the plates from Picasso, he published them in editions of 250 each.
“That was an unusually large number and revealed the faith Vollard had in his protégé. It was such a large number, in fact, that the dealer had the plates steel-faced (coated with a thin layer of steel) to prevent them from wearing out.”

Understanding, of course, that the Saltimbanque Suite did not include “The Tragedy”… I guess I should have said “Steel-coated” or “steel-faced”. 🤓

Bueller? Bueller?
Anyone else?
Pablo? Pablo?