I am aware of 1930 silver essai pieces, which I have one of; Bogaert-267B1
Also found as an essai in aluminum bronze, which I own an example of as well.
There were some 1920 essai pieces that were produced though. I have one and know of one other, but both are in nickel; Dupriez 182
So I imagine that there could be a silver essai out there.