Being embossed rather than incised, and the material being glass, it is obviously molded in rather than stamped. The following are general representations of the various backstamps, which may not look exactly the same on all shapes. Promotional patterns may have been available for as little as a holiday season or a year, standard patterns from two years to as long as a decade in a few instances.
The early clear glass Pyrex ware backstamp was a simple circle with PYREX in an all-caps serif font with Corning Glassworks' CG monogram above and below. The backstamp on the earliest color ware included the word PYREX with the abbreviation T. Other Considerations The earliest colored nesting mixing bowls have a deep base ring, the bases on later ones being almost flat by comparison.
The earliest #402 bowl was definitely more of an orange-red hue, described in advertising as "Chinese Red".
The red #402 bowl supplied with the 1970s Friendship pattern collection differs from the red 402 included in the #400 multi-color mixing bowl set, although it is similar to the shade of the earliest red 402s of that set.
Backstamps On the bottom of most every genuine Pyrex opalware article is an embossed set of markings which contains a variety of information. The backstamp seen on various items prior to that echoes a Mac Beth-Evans trademark, but one not previously used on their glassware: that of a glassblower, or "gaffer", nicknamed "Little Joe". Since pattern collections and promotional pieces were available for relatively finite periods, knowing their years of introduction and discontinuance can also help narrow down dating somewhat.