If originally a pressure tank, 2line, it is a 1959. If it has a fuel pump originally, it is a 1960 or later. In any event, the 5 1/2 is a plain bearing engine.
These are some things I keep in mind when mixing a tank of fuel:
*Modern TCW-3 oils no longer require 24:1. The recommended mixture changed in 1964, but the engine specs did not.
*Never use chainsaw oil in an outboard. Air cooled oils are different from water cooled oils.
*Add extra oil for a plain-bearing applications.
*16:1 will fog the lake for mosquitoes. Use only for antique chainsaws...maybe.
*Many Johnson/Evinrudes were labeled 100:1 during the 80s. I NEVER do that.
*With modern TCW-3 oils, especially synthetics, 50:1 works just about anywhere.
*Double oil, 24:1, is recommended in most operators manuals for continuous-full-throttle or 'racing' usage.
*Keep your eyes on what you're doing. Never mind the girl in the red sportscar.
Mark any fuel container with the fuel inside as soon as you mix it. I use combinations of white and black wire ties to mark tanks. White is raw gas, black is 50:1, 2 black ones for double oil...etc. Buy cheap, weak, wire ties that are easy to break off. They are just reminders.