The P91 pistol was produced in limited #'s from 1992-1994 and it had 11 round magazines.
After the AWB in 1994, Ruger started making the 10 rounders only.
When I bought my P94 and PC4 ,I grabbed up as many of the 11 rounders that I could find, since they are pre -ban in my state, (pre 1994), they are legal to possess. They are super hard to find today.
And .....40 cal Pseries guns.....P91,P94,PC4 all fit same magazines