The #120 is one of Stanley's most common block planes. Stanley produced the #120 between 1876 and 1950. They probably sold millions of them. I'm not sure it was the best block plane ever produced, but with a really sharp iron, and a medium to light cut, it can produce a very satisfactory shaving. It's a great worker, it's versatile, and a nice example can usually be had at a very reasonable price.
Jim C.