Ace Books' first series of paperbacks, the D/G/S series, began in 1952 and ran until 1965, by which time other series from Ace had begun.
The D/G/S series used a serial number from 1–599, and a letter code to indicate price.
D-series books cost 35 cents; S-series titles were 25 cents; and later there were several G-series books, priced at 50 cents.
Note that there is a separate G-Series, which began in 1964 with independent numbering from this series.