Prussia - 4 Groschen - Frederick William III - not silver

[$and€r]

Can this 4 Groschen be a contemporary forgery back then or should i look in other directions..
My knowledge for German States coins is minimal..
Any suggestions are welcome.. Thanks so much.. 

Composition = copper or so..
weight = 3,70 grams
diameter = 25/26mm

(N# 15622)

[FosseWay]

I'm not familiar with that particular issue or issuer, but in principle a contemporary forgery is probably the most obvious explanation. It would have been silverwashed before use, or potentially given its general condition, has been silverwashed and the wash has rubbed off.

Below is an almost contemporary UK shilling forgery which would have got the same treatment.

I believe there may have been a loophole in the law, or at least there was an urban legend among forgers that there was, that copies of silver coins made in obviously non-silver metal were sufficiently different from the real thing not to count as forgeries. The die sinkers would churn out quantities of "non-forgeries" in brass and sell them to individuals who would then silverwash them, pass them for the real thing, and take all the risk of being caught (which could involve the death penalty in some places).