I got this lens today, marked: RUO DOPP-ANASTIGMAT IRICENTOR 1:6,8 F=18cm OPT-WERKE RUDERSDORF-BERLIN No 2139 in an early working Compur. The "Vade Mecum" says it's a Dagor type, and "a really good design" according to W. Frerk. Most of the RÜO lenses I find on the internet have low serial numbers. I wonder how long Optische Werke Rüdersdorf were in business? The lens is very clean and scratch free, but has a light yellow tinge. Not much, only as an old UV-filter. What could that be? Yellowing balsam? The glass?