For the Single-Six, .357 Blackhawk and .44 Blackhawk, Ruger added the new barrel lengths in early 1959.
The first block of 4-5/8" Single-Sixes shows up at 133983, but there were a few random 4-5/8" guns earlier than that. I don't know if those earlier serial numbered frames just happened to get barreled while the 4-5/8" barrel lot was going through or not. The earliest known is 128605.
For the 9-1/2" the first block of those begin by at least 139036 with no known earlier random numbers.
The 6-1/2" Magnum Single-Six (this started out being offered as a separate series from the Single-Six .22 LR) was announced at the 1959 NRA show in March. The first guns were shipped in July 1959, with the serial number block beginning at 300000.
Chet15