PTGs was the answer to a New York Times crossword puzzle. The clue was "Series of book copies: Abbr." Obviously the word is printings. Enter ptg in Googe and you will see commercial printers.
PTFE - PTFL - PTFO - PTFP - PTG - PTGB - PTGE - PTGEC - PTGEWS - PTGIG
"ptg.." Abbreviations.com. STANDS4 LLC, 2017. Web. 24 Mar. 2017. <http://www.abbreviations.com/ptg.>.