Blank silver planchets that might fit a penny press would be a smaller dime flan. That penny was struck on a regular penny sized planchet. Do an edge test on a testing stone or jewelers file. I'd bet there's copper under that plating. My 84 yo dad says as kids they used to dip pennies in mercury to pass them off as dimes. Wouldn't recommend that today. Today, it's very easy to plate a penny with a few white metals & electrolysis at home.
Phil, they're real easy to plate. I electroplated them in high school. All you need is a battery and wires attached to the electrodes, and the chemical solution. I'll bet they even have it on youtube, if you need to see it. Because of that, these really aren't a "novelty." Do you know what I mean?