We used it to designate glass colors that were produced during the transition from one color to another in our continuous-melt furnaces. For example, if we were producing dark blue and wished to change the color to medium blue, a shade between the two might be made as the glass gradually changed color. There was nothing really wrong with the T-Glass, it was just lighter than one standard shade and darker than another.
Color is just one characteristic that goes into defining a standard.
As our quality control systems evolved so did our tolerances with regard to other glass properties. Each product was soon being judged for surface texture, mix proportion, light transmission and other qualities. When a glass fell outside the tolerance range, it also received a T-Glass classification. So T-Glass came to encompass any glass we make that, for one reason or another, falls outside the QC limitations for it's standard. Structural tolerances are a different matter altogether.