This insulator was used on a line built around 1910, which was insulated mostly with slightly smaller Victor glaze welded cables. This larger piece was used as a replacement were the uniparts failed, though I`ve found only 2 complete and the remains of around 10 total. What makes this piece interesting however, is that while it looks like another glaze weld cable, the two shells are actually cemented together, making it a multipart if I'm not mistaken.

I`m curious if anyone else has one of these and its M-Number. Dimensions are exactly 7" across by 4" tall.

Thanks to Dustin Logan, this is an M-2200.