Interesting they leave that out on ME on those years? By going to the Sierra parts break down it should be #303345 or Sierra #18-2022. 1" diameter. Hope that helps. That is using the parts from the gearcase rebuilt kit #18-2686 which list from 1958 – 1967.
I can confirm OMC #303345 as the correct number from when I replaced the bushing plate and seal on my 1958 Johnson 10hp.
Bob
1937 Champion D2C Deluxe Lite Twin
1954 Johnson CD-11
1955 Johnson QD-16
1957 Evinrude Fastwin 18
1958 Johnson QD-19
1958 Johnson FD-12
1959 Johnson QD-20
“Every 20 minute job is only a broken bolt away from a 3-day project.”
"Every time you remove a broken or seized bolt an angel gets his wings."