Once, all lumps were named like GAMENAME.??? (where ??? was a extension signifying type of data like TMN, TAP, or SNG). This was rather silly, like saying the name of your game several hundred times in quick succession. ARCHINYM.LMP was added to fix this.
ARCHINYM.LMP is a text file, which contains the Internal name used by all of the flexible-named lumps in the file. Then it has a newline, which might be either Unix or DOS style, and a string that tells the version number of CUSTOM.EXE that the file was created with (or the version of RPGFIX that first added the ARCHINYM.LMP) followed by another newline.
So. Read the first line of ARCHINYM.LMP to get the correct prefix of most of the other lumps.
If ARCHINYM.LMP does not exist, you know you are dealing with an obsolete file, and you can assume that the variable-named lumps will use the RPG filename as a prefix.
Since the addition of ARCHINYM.LMP, new lumps have been named arbitrarily, like DEFPASS.BIN or BROWSE.TXT. In addition, old lumps (GAMENAME.???) in new games will be named OHRRPGCE.???. This is a trend likely to continue.
ARCHINYM.LMP is one of the few text based lumps.