To elaborate on Bob's answer.
In 1953, Bill's SA design came out, to where you pulled the hammer to half cock, and the cylinder would rotate. You opened the loading gate to just load the gun.
I 1973,,, the New Model design came out,, with the safety transfer bar. And with the re-design, the manner in which you could rotate the cylinder for loading & unloading was changed. All Ruger SA's since then require the loading gate to be opened to rotate the cylinder.
So, for 20 years,,, it was one way, and now for the last 43 years,,, it has been different.