The original phone circuits here in New England (especially in rural areas) ran along 6-pin crossarms and these are among the earlier stuff (c.1890-1910) that Ma Bell constructed. As new subscribers were added 10-pin arms were mounted below and that was around 1910 or a tad bit later. Often the original 6-pin-arm poles were outright replaced when upgrading was needed owing to the aged poles and additional weight placed upon them. Here is a nice photo of an older pole with a longer crossarm added to it.