Each of these is individually serial numbered to less than 60,000 and branded "3M BRAND 17.22MM WOLLENSAK NO. E58791" (# is the one pictured). Front clear lens diameter is 7mm, barrel is 25mm. Rear clear lens diameter is 6mm, barrel is 17mm. The entire lens assembly is 23mm long and appears to be made entirely of brass. Multi-element with fully coated optics! A Wollensak collectible!