https://www.glassbottlemarks.com/diamond-i-or-i-inside-a-diamond-mark-seen-on-antique-bottles/
Scroll down until you find the picture of the diamond with numbers inside.
"“766” inside a diamond – mark on the base of a small “generic” light aqua bottle made by Illinois Glass Company. This identified the bottle as their “No 766” mold in bottle catalogs and/or communications with buyers.
There are also many bottles known with a number or number/letter combination within a diamond. These are also products of Illinois Glass Company, and some of the these numbers have been matched up with bottle style listings in early trade catalogs published by Illinois."