Recessed is not just the cylinder, but every chamber. A space is milled into the end of every chamber allowing the rims of each cartridge to sit flush with the rear face of the cylinder.
At first it was thought to be needed because the magnums were so powerful. They found out the cylinders were strong enough with the appropriate heat treatment. Gotten rid of as a cost cutting measure.
The pinning was supposed to keep the barrel from turning loose, but turned out to not be necessary.
