Captain America Marvelmania piece from the 1969. Probably inked by Jack. I also suspect he added the letters and pasted them on. The crafstmanship isn’t all that spectacular on the piece. You can see the stars at the top aren’t the same size and the circles have some sloppy areas, but this was for a tiny botton, so I doubt anyone would’ve noticed.
Here’s the button with several other Marvelmania buttons, all of which look like they were penciled/inked by Kirby.