I believe those “numbers in a box/circle” were Tupperware’s method of matching bowls, etc. to their respective lids. You will probably find the lids that fit them to have the same number in a box/circle.
Most plastic containers are marked (usually on the bottom) with a number within a triangle with arrows - commonly known as a recycling symbol. These numbers (ACC resin identification coding system) were created in 1988 to facilitate recycling programs across the country. These recycling numbers can range from #1 to #7, depending on the type of plastic. The #7 recycling label is a catchall indicator for plastics made with a resin other than those in the #1 to #6 designations, or made of more than one resin. The #7 category not only includes polycarbonate, but also includes compostable plastics made of organic material.
American Chemistry Council Resin Identification Coding SystemJim
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