Found this in a collection I recently bought. An OB multi (not sure of the M number) sand etched 407. There are 2 identifiable fingerprints under the glaze along with accompanying smears and smudges of the circular brushed glaze. It's not too often that you find insulators with actual fingerprints in the glaze!

Update: Thanks to 2 other ICON members, I have been corrected that this is not 407, but 4 with OB in between a 7. Possibly/ probably representing manufacture year of 1947 and their company mark of OB. Thank you both for this info!