1951. Progression from Summar > Summitar -> Summicron. Thorium lenses with yellow-ish tint reflection, which were replaced with Lanthanum (LaK9) in 1952. Some Lanthanum lenses up to ~1050000 were probably contaminated with Thorium oxide, so they also have a yellow tint. Note the extra "ring" above diaphragm numbers, with vertical lines. Serial numbers start with 920001 and go to 922072. There are two conflicting theories about the first batch. Some say 922001-922097 was the first batch of 97 lenses, and 920001-922000 range was used after, others say entire production line started with 920001.