"LS" == Laurent Siebenmann writes:

 LS> In the Microsoft Windows Russian Cyrillic encoding (ISO?) 1251

no, ms 1251 codepage is not an ISO standard

 LS> one finds 4 quotation marks that originated in France with Mr
 LS> Guillemet. The common-names and hexadecimal codes are:
[...]
 LS> Question: In the Microsoft DOS Russian Cyrillic "Alternative"
 LS> encoding as most usually sold, what are the corresponding codes?

the standard ms 866 codepage does not contain them; some variants of
cp866 do contain them.  probably because of this, guillemets are not
often used in plain text russian documents; but books in russian do
prefer to use guillemets.

the fact that cp1251 contains guillemets does not lead to using them
in emails and texts, because most russian keyboards do not contain
direct keys for guillemets.

 LS> Beyond "1251", "DOS Alternative" and "Unix-Internet KOI8", is
 LS> there any Cyrillic input encoding with a big user base?

ISO 8859-5 is used in many commercial unix flavors and in cyrillic
text exchanges abroad russia; MAC cyrillic encoding is used on Apple
Macintosh platform; asian cyrillic encodings like PT154 are perhaps in
use too.

Best,
v.