The first two volumes of this collection, of about 430 tunes, was found online at the National Library of Scotland. The single-tune files here have names of the form VPPPN_Title.abc, where V is the volume (1,2), PPP is the 3-digit page number, and N is the tune number on the page (usually 1, sometimes 2, rarely 3). VPPPN is also used for the tune's index (X:) number.
The full collection has a name starting with "ERM". There are two versions of a few tunes, with a suffix of "-V1" or "-V2", indicating the versions of ABC that are used. The V2 tunes use such things as voice overlays, crescendo/diminuendo symbols, etc. These features weren't in the earlier versions of ABC, which were designed mostly for simple (fake-book, lead-sheet) music notation. Some (but not all) of the common ABC apps can now handle these more advanced features.
Two complete ABC files are made: ERM-V1.abc and ERM-V2.abc. The V1 and V2 files are also distinguished by replacing the first '_' with '-' for V1 and with '=' for V2 files, so that the Makefile can more easily distinguish them when building the ERM-*.abc files.