Your age estimate is at least 10 years off. M-3 was pretty much not made after the L-100 and M-100 series were introduced in the very early '60s. The leg design indicates trying to match contemporary smooth leg, brass ferrule furniture from the mid-50s. I would suggest this was made prior to 1960 at least.
If you are certain that the organ has not run for 15 years, this may pose several problems. The oiling with proper oil may take days or weeks to travel through the hundreds of cotton wicks leading to the bearings inside the tonewheel generator. There is no "reservoir" of oil. It spreads from the funnels to the trough, and then on to the threads. There is also a square silver cup at the left side on top of the motor. Moisten lightly with oil.
This unit may have a curved masonite cover which should be removed for best service-5 or 6 screws along the top and bottom edge.
The electronics in this made use of big silver capacitors. I am no expert, but these are way way past their design life. There could be a leak, or one could pop and make a mess, or they could be okay for a while, but most likely-not.
The cord may be dried out and dangerous.
The correct start procedure is to flip and hold the start switch for the slow count of 8, followed by adding the run switch simultaneously for the count of 4, and release the start switch. I do not recommend that you try this without servicing the generator and the capacitors.
What are your intentions? Play it? Sell it? Value, even though this is a fine organ, is nil to a couple hundred or less IF in perfect rehabbed restored condition, near a metropolitan area.