transliteration of mjagkij znak (Cyrillic soft sign)

[Otto Stolz]

Hello,

I am wondering how U+02B9 MOFIFIER LETTER PRIME made
its way into the Unicode repertoire, and how it
acquired its comment “transliteration of mjagkij znak
(Cyrillic soft sign: palatalization)“.

ISO/R 9:1954 through ISO/R 9:1986 map the mjagkij znak
“ь” to the apostrophe, and so does DIN 1460:1982. The latter
clearly depicts the apostrophe that later became U+02BC,
while I am not sure whether also ISO/R 9 does so or rather
depicts a glyph like U+0027. (All of these standards
predate Unicode, so they just depict glyphs.)

ISO/R 9:1995 maps the mjagkij znak “ь” to the prime,
particularly to the modifier letter U+02B9, in accordance
with the comment in the Unicode charts.

Unicode archeologists, can you shed some light on the
history of both U+02B9 and the mjagkij znak?

And linguists, can you tell me how the mjagkij znak is
transliterated normally, as an apostrophe or as a prime?

Thanks for any comments,
   Otto