One of my favorite past times (especially when I've been grounded from eBay) is trying to nail down U#'s and/or manufacturers of a couple dozen or so NN porcelain signals that have yet to be catalogued. Love it when I finally get one right and get to move it from the "mystery" shelf and put it in it's proper place according to U# (sad huh?). Anyway finally got it figured out that this is a U_274B (new U# listing in "Value Guide for Unipart and Multipart Porcelain Insulators" by Elton Gish)