It's my guess that it was dipped in an acid. If it was really missing all or part of the clad layer, it wouldn't have splotches of copper here and there. Chris
That was my first thought also Chris. The obverse surface looks porous. My question is, if it was left in the acid long enough to eat away almost 0.3 grams, why is the copper layer on the edge not eaten into? The reeding looks normal.
I suspect it was first worn or ground down before acid was possibly used. If it had been only acid, there would be copper showing in the lower details as well as the fields.
It's a common-date counterfeit, meant to circulate. These are generally composed of copper which is then plated with a thin layer of white metal.