The riverboat ivory chip mystery has been solved. A jeweler in NYC had the riverboat chips made. It is also very likely that he made the famous Edith Head Ivory opera ticket necklace (that later sold at auction for over $300k)

We assume the 'riverboat' chips were recarved in the 1930s

I have newspaper articles and advertisements for the jeweler.

Below is an ad. Note he mentions English opera houses like Covenant Garden.

Also shown is what appeared in the Christie's auction catalog. Although the estimated value was $2000, it sold for over $300k