Lotta misinfo here.
On the older model S&Ws the one piece barrels were threaded into the frame - the cylinder gap was checked and the fixed sights were checked to make sure they weren't canted.
Then the barrel was pinned to make sure it didn't move.
This was before CNC milling machines were common and
handfitting of parts was still the norm.
The pin disappeared when they went to more efficient methods.