Damaged plate on transom bracket? Look on powerhead for a quarter size core like a small freeze out plug). It may also have the model and serial number imprinted upon it.
As Vic explained, the engine is a pre 1980 model due to the design of the model number as per Vic's second sentence.
There is no such model number that starts with the number of 83.
Model number 8599R is explained as:
85 = horsepower
1st 9 = Last digit of any year between 1970 to 1979, in your case = 1979
2nd 9 = No idea, lost track of that one. Usually prtains to short shaft/long shaft, something of that nature.
R = Factory production run, of no consequence.