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A 40 hp Mercury outboard, serial number 5897057, would have been manufactured in late 1981.
Mercury outboard, 8hp, sn A721731 manufactured in 1987.
The "OC" model 75 hp Mercury outboards were manufactured in 1990.
The 20HP Mercury outboard with serial number 9137627 was made in 1979.
The "OC" model 75 hp Mercury outboards were manufactured in 1990.
A 9.8 hp Mercury outboard, serial number 3264445, would be a 1972 year model.
The serial number 9137327 for a 20 HP Mercury Outboard is not not listed in the Mercury Marine parts catalog.
whats the serial number
From what I can find in serial number tables, your outboard is made in 1975. My refernce: http://www.marinshopen.se/Hj%c3%a4lpmedel/Modellguide/Mercury/3565Hk/tabid/107/Default.aspx
As close as I can tell, the 75 hp Mercury outboard, serial number 9436217 would have been manufactured in late 1985, since the 1986-on models, were six digit serial numbers with an "A" prefix.
A 9.9 hp Mercury outboard, serial number OB235459 is a 1988 model.
A 35hp Mercury outboard, serial number 6445947, would be a 1984 model.