Thank you all for your comments.President Kennedy's Cape Cod accent used this "ur" transition heavily. He pronounced "Cuba" as "Cuber", even in formal speeches.
Interesting, PaulQ. I like that. This must be common in creative writing.Because the author is limited to a standard alphabet, we cannot actually know the exact sound that the author was trying to replicate.
How such representations are written is thus a matter for the author, although such spellings are usually subject to fashion and convention rather than any particular true representation - the spelling may change over time.