> Is it theoretically possible to make a perfect rom set for an old version of
> Mame, from a perfect rom set of the current version ?


A perfect set, no.
The reason is simple: Rom sets change from version to version. As long as these changes are only new files being added or files being renamed, you'd be able to restore an old version perfectly.
But it's very common that old files get removed because they were wrong. They get replaced by good redumps. So you won't be able to restore these replaced files.
(ok..if you used the rebuilder or fix missing for adding new files, old files get moved to the backup folder, so you probably still have them)

The best start is to use the Rebuilder. Load a datfile representing a collection you want to restore (like MAME .36). Set the rebuilder source to the folder with your current sets (like MAME .111) and the destination points to a new folder. The rebuilder will now take everything valid (for .36) from the source and recreates it in the destination.


Roman Scherzer