I'd gamble on WM standing for Welsh Metre - the common Welsh Psalm metre 8787 being the most likely candidate.
Most of the tunes will be outside of HTI's current scope, which deals with "tunes associated with English-language hymns found in sources printed in or before the year 1820". Not only this, but there are very few publications of Welsh psalmody/hymnody before 1820, and it took a while for Welsh tunes to get into English publications (one reason being this very issue of metre - tunes that suit the Welsh language are often awkward in English. Several 18th-century tunes were still in common enough use to appear in publications in the first half of the 19th century - it's worth a look through Richard Mills' 'Caniadau Seion' of 1840 (an American edition of 1853 can be viewed online at the Internet Archive: https://archive.org/details/caniadauseionsef00mill ) - In this book, the metre 8787 is called M.C. Cymreig in the index (i.e. Welsh Common Metre)
I have pasted out from "Ship of Fools": ""According to the index of metres in "Caneuon Ffydd" Short metre 6686 Welsh short metre 6786 Common metre 8686 Psalm metre 8787 Long metre 8888"" So maybe... more
There's a Welsh psalm metre (Mesur Salm) which is 8787 but with feminine endings to lines 2 and 4. Alan Luff - Welsh Hymns and Their Tunes (1990) gives 'The King of love my shepherd is' as an English ... more
Thank you David, Finn and Sally You have given me a lot to think about. The Book is a manuscript. I will bring it next weekend. As far as deciding whether the tunes fit, for those of us with very... more
I look forward to seeing this fascinating volume next Saturday, especially the Scottish items. The 1790s seems early for American imports. As for the Welsh tunes, what a vast number there are in the... more
Another couple of things available online that might be worth perusing: Hugh John Hughes' "Y Drysorfa gerddorol" of 1857 - an American publication of Welsh tunes, with a handy index of authors at the ... more