I'd like to install particular version of libstd++6 with following command:

# apt-get install libstdc++6=4.9.0-8  -V
Reading package lists... Done
Building dependency tree       
Reading state information... Done

The following extra packages will be installed:
   libstdc++6 (4.8.2-16)
0 upgraded, 0 newly installed, 0 to remove and 216 not upgraded.

It gives no error, but apt keeps version that was already installed. And also it refers to this package as "extra".

There's no apt preferences set in /etc/apt/preferences.d. And the desirable version is definetely available through our local mirror. (If I try to run "apt-get download libstdc++6=4.9.0-8" it will download exactly desirable version.)

System info:

# cat /etc/issue.net
"Debian GNU/Linux jessie/sid"
# uname -a
Linux www27 3.13-1-amd64 #1 SMP Debian 3.13.7-1 (2014-03-25) x86_64 GNU/Linux.
# dpkg -l |egrep -i "apt|dpkg"
ii  apt                                   0.9.16.1                     amd64        commandline package manager
ii  dpkg                                  1.17.6                       amd64        Debian package management system

Any suggestions?