"Matrix" was my spontaneous idea, but my knowledge of the printing industry is rather limited. Your comment made me look at one of the more reliable websites which offer help and it's quite interesting (but not necessarily helpful) what I found:
http://dict.leo.org/englisch-deutsch/MaternI don't think I've ever seen the word "flong" before, but it does exist:
https://en.wikipedia.org/wiki/FlongI'm not too keen on "matrices" in connection with printing, but "printing template" is a common expression.