"OMC definately puts a welsh p
"OMC definately puts a welsh plug (small silver disc size of a quater) on all their engines. It is usually on the right side waterjacket (standing behind the motor) towards the extreme rear of the powerhead.
Sometimes though, it gets burried by all the "stuff" hanging on the motor.
Since sometime in the 80's they included both the model number and the serial number on most of them. If it only contains a serial number, it's of little value as far as determining year.
OMC (unlike most of the others) is good in other ways though. They stamp the part number on almost every part/piece on their entire outboard.
So if all else fails, if you find a part number on items such as the intake cover, exhaust cover (a major part) - this can sometimes equate to an exact year on the parts list (or at least get it down to a couple of model years)...."