To confuse the issue, for a decade there were two pay grades of SSG, E5 and E6. From 1948 to 1958 there were no SGTS (three chevrons); the lowest grade of sergeant was SSG E5. In 1958 when pay grades E8 and E9 were introduced, the other services just added two ranks; e.g. the navy's highest enlisted rank had been Chief Petty Officer E7, so they added Senior CPO E8 and Master CPO E9.
This wasn't confusing enough for the army, who in order to come up with nine enlisted pay grades, added one new rank (SGM E9) and revived the rank of SGT (three chevrons, no rockers) as pay grade E5. Thereafter anyone promoted to SSG was an E6. To preserve the prestige of the NCO corps, those already holding the rank of SSG E5 were allowed to continue to wear their old rank. For a decade someone with one, two, or three rockers might be one of two pay grades.