I believe the $20 chips to which you refer were all used in play. I believe they are rare because people in general did not collect chips back then, and so very few chips were collected and retained as souvenirs, with the rest eventually having been destroyed.

Providing the proceeding statement is accurate (and there are people more knowledgeable than me on the board who can opine), then the first $20 used in play on table games would be the Resorts chip, since as you mention it was the first casino to open.

If I am reading it accurately, according to James Campiglia's "The Official U.S. Casino Chip Price Guide" (3rd edition, although the 4th has recently been released), the Resorts $20 was the only Atlantic City $20 to actually be marked as a Baccarat chip. All others could have been/were used for Baccarat, but were not specifically marked as such.

I hope this helps

Brent J. Jensen
orbis non sufficit