These four are all of the variations of the Grand Trunk Pacific U-1366 and U-1367 (upper right) that I've been able to locate to date. The two green ink-stamped examples in the lower half of the image are typical of this marking and are available somewhat abundantly. The blue ink-stamp U-1367 at upper right is the second most abundant marking style I've encountered (I've seen somewhere in the area of 20-30 of these in collections and at shows). The black ink stamp U-1366 with hollow" letters at upper left remains the only example of its kind I've seen.
If anyone has seen an additional example of the black ink stamp piece at upper left, or any other marking styles (a few others are rumored to exist) I'd love to hear from you. They certainly didn't make just one! Their rarity to me may simply be because of how far away I live from where the Grand Trunk Pacific Railway ran.
Information in Mark Lauckner's "Canadian Railway Communications Insulators 1880-1920" (page 24-2) indicates that examples of the green ink stamp variety of the U-1366 were found at the site of a porcelain factory in modern day Czech Republic. The origin of the blue ink stamp U-1367 remains unknown at this time though I suspect they are also of European manufacture due to the quality of the porcelain. As they were used on the Grand Trunk Pacific Railway (completed in 1913), these are a fairly late-generation Canadian beehive style made in the earliest part of the 20th century.