If it has the external oil pump it's most likely somewhere around a 1995 or later. Like Jerry said, Mercury used these blocks on the Sport Jet version. These motors are almost identical to the Force motors, however, there are a few differences.
Pistons, rods, flywheel, ignition system etc are identical except for the lower crankshaft bearing. The Sport Jet motor has an extra gear on the bottom of the crankshaft to drive the oil pump. As a result, the lower crankshaft bearing has to be located just a tad lower on the crankshaft to make room for the gear. The locating pin hole on the lower crankshaft bearing outer race is drilled in a different spot than the one used on the Force motors. If you have to replace that bearing, the one for the Force outboards will not work. The bearing and seal are identical in all other aspects.
NOTE: I have been able to modify the Force bearing in order to get it to work. You need to use a Carbide drill bit (can't remember the size off hand) and drill a new locating pin hole. Regular high speed steel or Cobalt drill bits won't even scratch the bearing steel.
The other difference is there is a drive shaft that connects the gear on the crankshaft to the oil pump, and the oil pump mounting boss on the side of the block.
