You can safely assume all the holes on top are oil points, there are a few in connection with the bobbin winder, and sometimes there's one for the axle holding the hand wheel and stop motion screw. Sometimes the hand wheel needs to come off for oiling, especially machines comming out of long term storage. If there's a lid in the back, investigate parts there, all joints, gears, hinges, metal parts moving against each other needs a few drops of oil (assuming it's very dry). The same from behind the face plate and under the base; have the stitch lenth lever on maxium length, turn the hand wheel and oil end of rods. I'm sure you are onto it already. These models are often very close, and by searchin up model 15 clones you might find exactly your model under various names.