What year is this motor?

[Buck Horn Deer Farm]

I was given a free 100 hp Mariner outboard. I want to get a repair manual but need to know what year it is. There are no numbers on the mounting bracket or tag. The only number is on the back corner of the block on a disk (like a soft plug) with this number, C204507. Any help?

 

[racerone]

I would say it is a 1989 model.