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I think Bulgarian Ъ is more closer to short О or У then Ы.

So България transcripted as Bulgaria in Western languages and as Болгария in Russian.

It is not real image from old book. Only stylization.
Old cyrillic books have no spaces between words.

http://knigorod.raspopin.ru/gallery/vie ... =letopis_1

Real scans of one old hand-written...

Even more one case:
ЪИ ----> Ы

Добрый день! :D