Under the title "Studio Handbook" from S1 to S8, then "Studio Paperbacks." It's unclear whether an S13 was ever published -- the ISBNs for S12 and S14 are in sequence (see Ad, PW, Aug. 28, 1972, at 183 (listing the Fall/Winter 1972 titles, including S9 through S14 but not S13) and I haven't found any mention of S13 anywhere.
Viking Now Publishing Books from Studio Publications, PW, Feb. 9, 1959, at 85.