1978 Mercury 1150   Lower unit  20" shaft 

(Serial ID #shows as a 1978)

Photos show 22 1/4 approx from top of casting to tip of driveshaft   

8 splines Driveshaft

15 Splines Propshaft   

It's the common Gearcase used thru 70's early 80's     Check photos   Ask questions   

Thanks!