What does 6 represent in Medieval Welsh?

Question

In both A Glossary of Mediaeval Welsh Law (1913, Timothy Lewis) and Early Welsh Gnomic Poems (1935 ed. Kenneth Jackson) you can find the numeral 6 used (apparently) to represent a vowel. Can anyone explain this?

I’m guessing it might be representing some particular form in the manuscript. Luckily for my Google-fu, this same example appears in both (pages linked above):

a phan edrych6yt ar y dyle nyt oed arnei namyn byrwellt dysdlyt ch6einllyt a boneu g6rysc yn amyl tr6yda6,…

Edit: After Yellow Sky’s answer I found that this and some other characters exist in Unicode.

• Ỻ U+1EFA LATIN CAPITAL LETTER MIDDLE-WELSH LL
• ỻ U+1EFB LATIN SMALL LETTER MIDDLE-WELSH LL
• Ỽ U+1EFC LATIN CAPITAL LETTER MIDDLE-WELSH V
• ỽ U+1EFD LATIN SMALL LETTER MIDDLE-WELSH V

a phan edrychỽyt ar y dyle nyt oed arnei namyn byrweỻt dysdlyt chỽeinỻyt a boneu gỽrysc yn amyl trỽydaỽ,…

Answer 1

It is obvious that 6 stands for the modern letter w which in Welsh can be pronounced in some words as the consonant [w] and in other words as vowels [ʊ] or [uː]. In your example there is the word byrwellt in which w is the consonant [w], while 6 in your example stands where w is a vowel letter.
Your example is from Breuddwyd Rhonabwy ("The Dream of Rhonabwy"), a Middle Welsh prose tale, here your example text is printed with ws instead of the 6s.

Frankly speaking, it is the first time I have seen such usage of 6, and have no idea why they did it so.

UPDATE: I have just found an explanation in this book, in the very bottom of the page in a footnote:

Transcriptions of Welsh often use '6' for 'w' which reflects the form of the letter found in the manuscripts.

The next page of the book contains a passage in Welsh in which most of ws are written as 6s.

UPDATE 2: Here is a page from The Red Book of Hergest (Welsh: Llyfr Coch Hergest), a large vellum manuscript written shortly after 1382, where I highlighted some of the instances of w written as 6: