20hp tiller motor w/ no serial #

[sgms18]

I have a blue and black 20hp mercury tiller motor(I'm guessing mid 70s to early 80s) and the serial # tag on the left side of the lower unit is no longer there. I have been unable to find a S.N. on the power head or anywhere else. How do I find parts with no S.N. or no idea what the year or model is?

[boats.net]

If you have a Mercury dealer in your area that has been selling Mercury's for some time then they may be able to help you identify it.