Below you can see a pinned and a non pinned barrel. The pin is located at the top of the frame just behind where the barrel enters the frame. The 29-2 on top has the pin while the more recent 629 below it, does not. Note too that the M29 is a carbon steel gun which has been nickel plated while the 629 is stainless steel. S&W used the "6" prefix to identify a stainless steel revolver.
Here you can see the recessed cylinder of the early 29-2.
Below is a non recessed cylinder of an early S&W in 44 Spl.
S&W recessed the cylinders of all guns in .22 l.r. and the magnum loadings, .357 Magnum and 44 Magnum for example. But did not recessed the cylinders of non-magnum guns. Others have explained when the practice ended. Better heat treatment made it unnecessary and it saved them money to delete it.
Get ahold of a copy of the "Standard Catalog of Smith and Wesson" which is a useful reference to have. The 3rd edition is the latest and can be found used for a lower price.