Up to about 1970, Johnson OB model numbers were two or three letters and a two digit number coding. Motors after 1960 of the 'RD' series were based on a 43.9 cu-in. block. The 'S' may be a production run code, useful for ordering parts. Guessing that the 'L' may be for long shaft, but this is all I can find.
Looks like we need Pappy to chime in here for the absolute answer ...
Good job, Dale! Right on the money.
The "S" meant that it had the newly modified lower crank seal in it. Prior to the "S" models there was a high rate of failure although most were repaired one way or another.