1969 Merc 650

There are "plugs" on the lower unit marked FLUSH and VENT, and then a plug on the very bottom of the motor which I assume is the drain, but which is the fill? What is a good way to fill it?
Thanks

 

You're talking about the lower gearcase oil, I presume?
My guess (I do NOT have any firsthand experience with a Merc of that vintage) would be, start by taking out "vent". There should be some oil behind it but it won't trickle out. Now, "flush" should lead to the water pump pickups- if it does, close it up and try the one you think is the drain. With "vent" open, oil should come out of the drain. (I put a 1-litre plastic tub under it, and use a piece of masking tape to direct the oil into the tub so it doesn't drip on my driveway.)
To fill the gearcase, put the nozzle of the oil bottle in the lower hole and slowly squeeze until the oil reaches the top hole. Put the top plug back in, then pull out the bottle and quickly put the bottom plug back in. Spin the gearcase around a bit (turn the flywheel and/or prop, with spark plugs unhooked). Now check the level in the top hole and add a few drops if needed.
You fill from the bottom so that air can't get trapped in there. If you fill from the top, the gears won't be immersed when it looks full, and you might damage the gearcase if you run it.