~~~ as seen on the octagonal which has an "F" on the 50.

The lower denominations sometimes did not have a monetary unit engraved, which were sort after for re-sale, either by a casino or via a manufacturer.

What is unusual about the "20" is the embellishment carvings on the edge and rim design which could be from the new owner which is not uncommon, especially with MOP originating in France and España that was sold to Cuba, Argentina and Uruguay.

I have seen recycled inventory of MOP in different stages of the process for a new home.

Good observation; I did not notice.