Phone openwire (at least here in eastern Massachusetts) looked a lot like this. I truly enjoyed looking at the side street stretches openwire subscriber leads along many side streets in the city I grew up in. By 1966 most of this old stuff was replaced by modern paired cable. The folks at New England Telephone were indeed very helpful and cordial as I poured through the small piles of insulators removed from their crossarms. Even though this was within So. Mass. Telephone territory, I never found any insulators marked as such. When NET took over SMT in that city during 1938 they rebuilt most of the existing openwire and did away with all CD 102s, 104s and 106s in the process, replacing them with clear glass such as CD 115s and 122s. They were very thorough about that! Anyway, I really never found anything spectacular in the phone company salvage yard. I think my best piece was a crown embossed CD 121 Am. Tel. & Tel. Co. I still have that one in my collection :-)
PS...Harwichport is on Cape Cod.