That chart was published by Owens-Illinois in 1964.
I noticed on that same chart there is also another instance of a mark that was presumably no longer in use by 1964…….
However, I find it very hard to believe that they would have continued to use the WT triangle mark all the way up to 1969 (when Armstrong was purchased by Kerr), as stated in some sources.
the “LM inside an oval”, by Latchford-Marble Glass Company, which was reportedly discontinued in 1957 (in fact, the name of that glass company had been changed to Latchford Glass Company).
I think Owens-Illinois was either somewhat slack about making sure their chart was up-to-date, or they merely wanted to include certain well-known marks that might still be encountered on relatively-recent glass containers made within the last decade or two of the publication of that chart.
In 1848 the name of the firm became “Whitall, Brother & Company” after Edward Tatum became involved.
In 1857 the name was again changed to “Whitall Tatum & Company”, and finally in 1901, to “Whitall Tatum Company” which was the name used until 1938 when the works were bought by Armstrong.
However practically all “W T & CO” or “W T CO” marked prescription bottles found in a very dark purple color have been irradiated (color-altered) and were clear to begin with. (Confusingly, many Whitall Tatum telegraph insulators, especially the “WHITALL TATUM CO No.