If it is an Akro Flintie, it will glow on the end of a penlight and you may get a fisheye effect somewhere through the glass as you turn it around on top of the light (not UV, just the red-dish color, Flinties are opalescent). Akro made ¾ inch Flinties so .70 is not out of the question.
The big Vitro gamers in that Art Deco box are over .85 or 7/8”. It does resemble them. The ones that I have will glow around the outer edges, and even through the middle on some. They are not as opaque as they look in hand either, no fisheye with these however, no opalescence.
I am betting that it is a Flintie, backlight it with a small intense light—smaller than the marble and spin it around. Or cut a hole in a piece of cardboard and backlight it with what you have laying around for intense light.
Marble—On!!