I think it's calling for either tongue-and-groove (first term) or mating (male-and-female) flanges--two similar but not identical pipe flange facings.

"The "male-and-female" facing consists of a recess in one flange and a corresponding raised face or projection on the other maiting flange....In the tongue-and-groove facing, the tongue or raised face and the groove or recess are narrow rings located between the bolt holes and the port. The male-and-female and the tongue-and-grove facing have been extensively used, particularly on hydraulic lines."
Standard handbook for mechanical engineers
http://books.google.com/books?id=QrQQTTmr3sQC&pg=RA3-PA148

A picture is worth a thousand words:
http://books.google.com/books?id=q8hBAAAAIAAJ&pg=PA81