Have you checked the actual motor plate or serial number...? I don't know where Mercury/Mariner installs their nomenclature plates, but on Johnson they are installed in the front of the engine mount. Also Johnson has a welch plug on the top of their engines with info as to the motor. Maybe Mercury/Mariner does the same things...
Is this your engine or just one you took a picture of on some other persons boat? If its yours check the serial numbers, they should be on the transom bracket.
If I had to guess the year I would say late 80's to 90's.