[thecatspajamas]

It's hard to tell from the scans, but does the pin actually fit into the back of the button, or was it somehow adhered to the back (resulting in the mess-of-whatever visible on the back when it popped off)? My eyes could be playing tricks on me, but it almost looks like a disc of some sort that someone had glued a pin onto in an effort to make it into a pin/button. If originally a pinback, I would find it more likely that the pin portion would fit into the back of the pin, within the lip of the curl, rather than being glued onto the back.

I also think a closer-cropped scan would be more helpful/clear (we don't need to see the empty space of the entire scanner bed).