This question should be asked rugerforum.com. The folks there are real Six series experts and some of them worked for Ruger in the heyday of the Six.
As I recall, the story goes that the Security Six was originally intended to be sold strictly as a .357, WBR saw no point in producing a nearly identical gun at the same price in a weaker cartridge. However it became apparent Ruger was losing LE sales because so many agencies did not authorize .357's, especially the lucrative NYPD. Also, adjustable sights were an extra cost that many agencies didn't want. So they shortened the chambers to .38 Special depth and added simple sized sights, thus creating the Service Six. A round butt was added later creating the Speed Six. Of course, either gun could also be had in .357. Interestingly, Ruger did occasional runs of .38 Special Security Sixes when requested.