mercury model 200 lower unit

[luvngspnful]

I need to know if a 1975 vintage mercury 20 hp lower unit would fit my 72' model 200 20hp power head.. I have checked ser# and they list as 72 and 75 both 20 hp. I assume the 75 is a model 200 also. I dont know anymore details regarding the 1975 model except they list it as a 20hp. thanks for your comments.
Dave