Mercury Lower Units

[Mike8509]

I have a 1972 Mercury 50 HP (500 Series) and my lower unit failed. Lower unit Series 1643. I am looking at lower units and I wanted to know what units would be compatible for the engine. I am assuming only the 1643 series but does HP matter. Does is have to be a 50 HP lower unit?? I have looked online for days and you would think there would be a compatibility chart or numbers to go along to identify lower units that would match up!

So if anyone has any info that would be great!:D