Elton Gish gives an answer: the U-55 drawing from Jack Todd is taken from an Illinois catalog, and it's quite the same as the Locke version. The U-55 has a narrower space between the wire grooves, and a definite band around the top. The insulator pictured above is a U-55.

I'm no expert in U numbers, and I'm trying to figure out exactly what this particular insulator is. Any ideas?

For comparison, on the left is a NO EMBOSSING U-52A, (I believe) and on the right is a FEDCO U-47 (as I said, I'm no expert in U numbers, so I could be wrong on those). In the middle is an incuse marked (on top) FRED M. LOCKE / VICTOR, N.Y. -- and this is the one I'm trying to figure out the U number on.

It would appear to me to be a U-55A -- the dimensions are about 2 1/4 by 3 1/4, which would seem to match the U-55A better than the U-55. And the profile appears closer to the U-55A. On the other hand, the Unipart Value Guide lists the U-55A Locke only in white, while the U-55 Locke is in brown.

Links to other views: Broken Side [id=203540278] Top View [id=203540197]