In the low back series both the adjustable sight and fixed sight revolvers were marked Security Six. Then when Ruger wanted to get into law enforcement sales, they changed the name of the fixed sight model to Police Service Six.
The Speed Six, from all info I could dig up, has always been marked as Speed Six, whether they were low back or high back frames.
Like Hittman (and several others) I also have a "lettered" round butt Police Service Six. Some of us that have one did the comparison of the frames (the PSS and the Speed Six frames are different in the butt area) and the ones marked PSS ARE built on Speed Six frames not PSS frames that had been rounded. Best theory was they did it to finish a contract and they didn't have enough PSS frames.