Does anyone know anything about the hymn meter numbering system found in Welsh books such as Y Caniedydd, Welsh and English Hymns and Anthems, and Caneuon Ffydd? As well as the usual way of describing meters, these books share a system of numbers from M1 to M100 (with some gaps in the books I've got) describing various meters. So M1 is Mesur Byr, i.e. Short Meter, and M100 is 8.8.7D. It's clearly been devised with some care – for instance, it distinguishes between M5 (which is Long Meter) and M21 (its evil trochaic counterpart, chiefly found in the carol Awn i Fethlem). I imagine this scheme is Welsh in origin (and not just in popularity), as M2 is Mesur Byr Cymreig, Welsh Short Meter, i.e. 220.127.116.11, which would be a very unusual meter in a non-Welsh context to think of at all, let alone list it ahead of Common and Long Meters! Does anyone know the origin of this system?