The only type 21 I've ever owned or used was a 9-1/2 block plane.  With a little tuning it worked just as well as my pre-war 9-1/2.  I still have a couple blue type 20 block planes in the till.  

One of these days when I find a cordovan at the right price I'm going to give it a tune up and see how it does.  I've tuned and restored many type 19s.  They can be a bit rough around the edges as I find Stanley's fit and finish department was a tad drunk on the job but with a little love they are very fine performers.