Cobalt blue glass? I have a few pieces that have numbers on the bottom.
What kind of cobalt blue glass? How are the numbers put on?..etched, impressed in the mold or raised?
Early opaque glass ( AKA milk glass) was 'marked" with a number on the bottom of the piece. And this opaque glass also came in a blue...but more of a sky blue than a cobalt.
How about a picture?