The "SM-NO" stamp on the bottom of these pedals refers to the date. For example: the stamp "SM NO 48 5" is the 5th month of the 48th year of the Shōwa era. The Shōwa year count in Japan started in 1926 (= year 1), so year 48 is 1973.
(Thanks to mondo500 on the D*A*M forum for this info!)
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