Provided it started out as a high quality socket and it didn't get hot when it was ground to shape, I think the 20 mm one should be just as good as a purpose-made bought one. I'm not so fond of the 21 mm one, I'd do that one differently (use a flat piece of carbon steel, end mill a slot, weld to the back part of a socket, harden and temper with a gas torch). My reason is, to make the flat parts that do the driving massively resistant to spreading. If the torque requirement were high enough, Bob's would spread.
If you don't have a milling machine, you can do the same job with a drill and a file, it just takes a bit more time and care.
