Smith and Wesson has used at least two systems on revolvers that have adjustable rear sights:
1. The front blade is separate, and pinned to the ramp.
2. The blade in integral to the ramp, but the ramp is pinned to the barrel rib.
Usually, but not always, the pin(s) were polished flat, and can't be seen except under close examination.
Front sights on fixed sight models are often part of the barrel forging, but again there are exceptions. Pinned front sights will often be found on non-standard barrel lengths.