hi, nice find. I have one, glass, 5 3/4" X 5 3/4" , teeny little glass marbles. not bullet mold. marked 'Standard Toycaft products co. Ny Ny'. Inside the little mosaic book that came with it, copyright 1937. all marked made in NY. It says "Your glass marble tray is four sided tray on one side, and 8 sided tray on the other. this gives you a chance to make 2 different kinds of designs, square and octagon".
It also says 'Mosiac designing is one of the oldest forms of art work and one of the most interesting, too." "you will find it really great fun".
EMK