At the end of one of the pieces is rubric which I read as:
mi brikiais yn gynta mak mwn hir/ a mak mwn byr/ a mak y delgi / a ffedair kaingk arhigain ymhob un o honynt // ag wedi hyni y pedwar mesur arhugain ar un gaingk ond nwydiu / r / mesur fal i gwelwch
or in a version by a modern Welsh speaker,
(mak mwn hir, mak mwn byr, and mak y delgi are three of the "twentyfour measures", which are the "chord sequences" on which the music is built.)Mi bricais yn gyntaf Mac mwn hir, a Mac mwn byr, a Mac y delgi, a phedair cainc yr hugain ymhob un ohonynt, ac wedi hynny, y pedwar mesur ar hugain ar un gainc, ond newidio'r mesur fel y gwelwch
My best version so far is
Am I close? (I'm doing this through dictionary work, and while I'm learning a lot about the Welsh language, I certainly don't speak Welsh!)I wrote first mak mwn hir, then mak mwn byr, then mak y delgi, then the twentyfour measures each one of them, and after this "the twentyfour measures, with the same cainc", but ?newer? measures ?can be seen?"
And, does anyone have any better suggestions?

So congratulations!
Da iawn i chi!