What year did they change the way HP is rated?

[Roughneck1860]

I wondering what year Mercury went from crankshaft horse power rateing to prop shaft rateing. I have a chance to pick up a couple early 80's motor's that are in near mint shape. One is a 2 cyl 40 and the other 4 cyl 50. I'm just wondering if they are the same motors as the 35hp and the 45hp "Classic Fifty" that were around in the mid '80's

Thanks
Tim