Mexico 1955 20 Centavos Off Metal Error

I've come across this 20 Centavo coin. The supposed error at hand is that it should be copper but it looks to be made of copper-nickel or silver. It is in a sealed plastic cover with the writing, "Centro Numismatico Compra Y Venta De Monedas" and an address and phone number which makes me assume it came from a Mexican coin shop at one point in time. Id test the weight but don't want to cut open the plastic as it seems quite old. Does anyone know what it might be worth or if more exist? Any knowledge on the topic would be appreciated!