You're looking at a comparison between the piece in question on the left, and a 'normal' CD 121 AMTEL [010] in lime green on the right. The skirt on the left measures a hair under 9/16". The skirt on the normal piece to the right is merely 3/8 +/- 1/16" The question is: How uncommon is this thicker skirt piece? This is the first I've seen in 10 years of (not so serious) collecting.

The consensus among a few well-regarded collectors who have emailed me over the past few days shows that this is most likely a Brookfield product. Brookfield made a lot of thick-skirted insulators as well as many more normal-skirted ones as well.