That one had me puzzling for long, as I thought the filenames had no direct relation to the database entries. I started mapping entries in the database to filenames and saw iterating filenames with some gaps between them.
I already discovered the MakeFid macro : record_nr << 4 | FILE_TYPE
but I thought that would make the ordering of filenames iterating without gaps...

Then the quarter fell. TAG-INFO OF DELETED FILES IS STILL IN THE DATABASE.

So the MakeFid macro works : record_nr << 4 | FILE_TYPE
or in other terms : ( record_nr * 16 ) + FILE_TYPE

recordnumbering of playlists/tunes starts at 16, as the root-playlist is named 10[01]