so we all know this website on lens aging

some random qns.
what if the date code is not stamped on the mount (or anywhere on the lens), is there a way to still tell when the lens was mfg?

I noticed that all the lens that i have seen are mfg at Utsunomiya ( U = Utsunomiya, Japan)

none from F = Fukushima, Japan
O = Oita, Japan

anyone knows why?