US pressings matrix numbering details
A hint to the pressing plants that handled Atlantic is given by the suffix to the matrix number on the label. These include:

PR = Presswell Records Mfg. Co., Ancora, NJ - they handled most of Atlantic's LP's
LY = Shelley Products, Huntington Station, NY
SP = Specialty Records Corp., Olyphant, PA
MO = Monarch Record Mfg. Co., Los Angeles, CA
PL = Plastic Products, Inc., Memphis, TN (mostly 45's, alas)
RI = PRC Recording Corp., Richmond, IN
AR = Allied Record Co., Inc., Los Angeles, CA (after WEA acquired the plant in '78-'79)
And Columbia, which pressed LP's for Atlantic (only available via the Columbia Record Club, a.k.a. Columbia House) up to 1973 and after 1978-79

Many thanks to Peter Reid of Timelapse Records, England for supplying this information via a contact of his in North America.