Square D Co. is one of my favorite manufactuers of insulators. For a number of years they made insulators for other companies under contract and usually marked with there logo. Hubbard & Co. T. C. Whites and Automatic Electric Co. were some of their loyal buyers among others. What is hard to find is the pieces they manufactured for themselves. I have several small spools and these strains that were manufactured for themselves. They cataloged their pieces a little different with an extra number. I have numbers 5500, 5502 and 5506 but haven't found a 5504 yet. We know of two pintypes made by them, U-32 number? which is always in shades of cobalt blue and U-317 number 5013 in white and brown, and possibly U-280B in brown. I have a U-280B which is similar in material to other Square D pieces but a shame it's not marked. Could also be other non marked pieces but have yet to find any evidence.