I read elsewhere that the f1.5 sonar renders a different look then the f2 sonar. I don't see it but I rarely shoot wide open.
The f2 is six elements and the f1.5 is 7 elements added in the front right?
The rear cell of the f:1.5 is a triplet, in the f:2 it is a doublet. Both the f:1.5 and the f:2 have four internal surfaces, this was important before lens coatings were widely applied.
P.S. "sonar" is an underwater locating system using sound. "Sonnar" is the lens; it comes from the German 'sonne'; sun.