It's a Rub Off insert from the 1966 Topps set. It becomes the second oldest card in my Williams player collection (which is made up of cards other than the regular Topps cards). It's in really nice shape; I've seen others of these that get all cracked and ugly looking.
The set was made up of 100 players and 20 team pennants. Other Cubs included Ernie Banks and Ron Santo. There isn't a single one of either of these on ebay right now. If an Ernie Banks pops up I'll go after.
Since this is a rub off, that means it reversed. I don't have any plans to actually rub it off, so through the magic of photoshop, here is what it would look like if it were rubbed off on a piece of paper.