Sounds like an early OB to me ... as best I recall it is that prior to 1960 OMC s/n’s were based on the block, through ~1970s the model numbers were 2 or 3 letters and a 2-digit number, where those #s identify the year of production, but not necessarily the calendar year.
After 1970 the OMC model numbers included the HP and year of Mfg’r (built in as a code). For example - J33ELUTA:
J = Johnson
33 = HP
E = Electric start
L = Long (20") shaft
UT = Year of mfg’r in code, see below
A = Production run (sometimes I've heard this last letter many not be there ... ? Pappy will know for sure!)
The 2-digit year code uses the letters of the word "INTRODUCES" to represent the digits 1-0. So the UT would be 73.
I N T R O D U C E S
1 2 3 4 5 6 7 8 9 0