I first found this Walker China pattern at a Goodwill last year, and I felt so lucky to find 3 more pieces at another Goodwill last weekend, for a mere .49 each. They just happen to be bread and butter plates - my favorite size.
It has taken me long enough, but I´ve stumbled upon the whole wide world of date codes - the numbers and letters used by companies to date their products. In this case, the 12-42 on the back stamp pictured above means these plates are from December, 1964. Ohh, delight! [source]