From: guto rhys
Yes - quite correct.
These changes, however, pertain to the 'transition' period between Late Brittonic and Early Welsh rather than to Middle Welsh (10th-15th centuries) - which is what threw me. Syncope and apocape of the Brittonic noun were probably complete by the middle of the 6th century. ('When did British Become Welsh' by Ifor Williams and Jackson (is that where you are quoting from?) deal with the issue.
Some personal names are an exception, giving us a few pairs: one of which reflects the form with apocape, the other having retained the vowel eg.Tudri / Tudur (Tudor) <*Teutorix (I believe the exact form is not attested).