MAME uses a parent/clone relationship to save space. For example:
If you download a clone from an index without the parent, the game will not run. index of mame roms
When people search for an "index," they are usually looking for a list of available games. However, in the context of MAME, an index usually refers to a Dat file or a specific organizational structure. MAME uses a parent/clone relationship to save space
Provide concise field list suitable for schema generation: If you download a clone from an index
Use torrents from reputable private trackers or archive.org. The best version to look for is a "Merged Set."
Search for MAME 0.270 ROMs (merged) on archive.org. These are legally gray, but the site has better legal standing as a library archive than a random index of server.