These commonly come out of *Arc.dat/*Dat.arc and sometimes compressed/pre-decompressed ARC files. It appears to be metadata... Some files don't have this but for files with more than 1 file, this has to be used to tell the game the proper offsets of multiple files in a binary. No clue what the header means... Doubt its from Nin, probably Paon.
3bytes: 46 4A 47 (FJG)
1byte: 50 (P [Paon?])
1byte: Raw file count
2bytes: Offset of raw file 1 (end of FJG header)
2bytes: Bytesize of all raw data combined