Carl Zeiss Jena Sonnar Electric 135mm f/3.5 – 5th Generation – 4 Digit Serial Number
This took a while to hunt down. According to all the usuals, this is the best variant of the lens for use as a tube lens. It’s confusing, though. The “Electric” version was not produced in great numbers yet it can sport a serial number of 4 digits to 7 digits. The 4 digit one is the money lens, though, so with that in mind, I eventually sourced this one. Not bad shape for a 40-50 year old lens!