I`ve had a look at the interesting article to which Tim helpfully drew attention, and wonder if your request back in 2010 brought any answers on the identity of the LM tune for Iam lucis orto sidere. It certainly looks like Fr Gordon was drawing on our local fiddling tradition.
Our church will have several good fiddlers in action for Easter Sunday from the Banchory Strathspey and Reel, and I could see if they know the tune, if you haven`t yet identified.
On the organ installation for Dufftown mentioned elsewhere in the article, an organ researcher friend thinks the RC chapel had an organ in the 1830s, so the installation of a "magnificent" one in 1845 likely meant this could be transferred elsewhere.
I have to report that the LM tune in Fr Gordon`s manuscript is not known to our Banchory fiddlers. The former leader of the Strathspey and Reel has hunted without success in his stacks of tunes. I`ve ... more