My 2nd and 3rd Editions of The Old Outboard Book shows the RDS-20 as a 1958, and the RDS-21 as a 1959, as does Marineengine.com.
Maybe I need to get a newer edition of the book? The 1959 and 1960 would easily interchange covers.
Dave
what Garry wrote here was based on the serial nr. only ??
or………..
😎
Opps – my mistake! I seldom look at the serial number for the year on OMCs. Garry is right. It makes a bit more sense now.