These appear to be related to either molds or production runs. On many pieces, various numbers and sometimes letters are seen in and around the backstamp. Later pieces are also recognizable by, instead of 'MADE IN U.S.A.', the the wording 'by CORNING, Corning, NY, USA' with the verbiage NO BROILER OR STOVETOP or, later, BAKING AND MICROWAVE below. Starting in the mid-1970s, equivalent metric capacities were also embossed on pieces, therefore any seen so-marked can be dated positively later than that. Sometime in the late 1960s, the circular configuration of backstamps on refrigerator dishes gave way to a more straight text format consisting of PYREX Р’В® in larger letters with model and capacity information in smaller characters above, and MADE IN U.S.A. A model number and, later, the capacity in pints or quarts were added above, and OVEN WARE below. 1950 or shortly thereafter, the registered trademark symbol 'Р’В®' was added below the name, the encircling wording became TRADE MARK above the name, with MADE IN U.S.A. No model number or other information was included on the earliest pieces. On some pieces, this was also enclosed in a ring. in a downward curve below forming a broken circle of sorts around the name. The backstamp on the earliest color ware round bowls included the word PYREX with the abbreviation T.M. The following are general representations of the various backstamps, which may not look exactly the same on all shapes. The configuration of the backstamp would undergo a few revisions after the introduction of opal glass kitchenware in 1945. The backstamp seen on various items prior to that echoes a MacBeth-Evans trademark, but one not previously used on their glassware: that of a glassblower, or 'gaffer', nicknamed 'Little Joe'. But the product would not be branded Pyrex until the debut of kitchenware made from it nearly a decade later. The plant there would be used to produce a more durable messware for the military.

Production of opal ware commenced in 1936 after the merger with MacBeth-Evans Glass Co.

The CG monogram is often mistaken for and referred to by collectors as a dollar sign, albeit a backwards one. below.Р”Р„ revised backstamp, with PATENTED above PYREX and below, was used after that date through 1924. 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. Being embossed rather than incised, and the material being glass, it is obviously molded in rather than stamped. While technically a trademark, it is most often seen referred to as a backstamp. On the bottom of most every genuine Pyrex opalware article is an embossed set of markings which contains a variety of information. While colors and styles of decoration reflected consumer tastes at the time of production, the first thing to look at is the backstamp. Estimating the age of Pyrex opal glass kitchenware can most often be done by observing a few basic characteristics.

Pyrex cookware