This is one of two known authenticated and certified Lincoln Cent off-metal prior to 1916. It was recently featured in a front page Coin World article and described as a possible Mint Experiment Test Piece. This was struck on a full-size planchet of Nickel composition.
Pollock lists as #2028, "Nickel. Plain edge. Unique? Listed in Judd as being a Mint Error."
S&N Labs analyzed this off-metal for elemental composition using scanning electron microscopy with energy-dispersive x-ray spectrometry (SEM-EDX). The composition was a copper-nickel alloy.
This is the same alloy that was used to strike the Buffalo Nickels during this time period. To view a unique 1920 Buffalo Nickel struck on a full-size copper planchet, authenticated and certified by NGC, click here.
This 1915 Lincoln Cent is on a full-size planchet as the rims are full and sharp. If it was struck on a foreign planchet, there would be weakness in the rims.