Patrick Hefferson asks how you can tell which AT&T tolls (CD 121's) were made by glass companies in Denver... the so-called "Denver molds". Woody answers....

The CD 121 was the standard toll line insulator for American Telephone and Telegraph Company since the first long-distance lines were built in the early 1880's. The overwhelming majority were manufactured by Brookfield at first, then by Brookfield and Hemingray. They were the standard for that service until about 1911 when the CD #152 was adopted. 

It is true there is a scattering of "misfits" which collectors prize today. Unfortunately manufacturer's identification is difficult. Insofar as the Denver area is concerned, we have the light-to-medium purple pieces, some of which were not embossed. These are almost an exact copy of the Hemingray version, with slight engraving differences. Note particularly the ampersand! I know of no one who has verified a manufacturer for these. The uniform purple color would rule out most sources. It doesn't even make a good match for the purples found in W.G.M. and earlier Denver production. More important, the mold characteristics are totally different. 

Otherwise, there are a few "misfit" Am. Tel. & Tel. in the aqua range. I am not aware that any of these have been tied to Colorado production by original location or for other reasons. I would certainly want to know this if it is the case!

On the other side of the coin: tens of thousands of Denver insulators (W.G.M. and previous markings) were everywhere in the Mountain States Telephone territory 50-75 years ago. This certainly indicates that local insulators were used to a great extent in favor of those marked A.T.&T. Co.