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Begin NCO numbering and pre-release explanation:
Posted to NCO groups in Spring 2010; 
Updated March 2011, 
Updated February 2014
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Users who want the latest NCO features before official releases,
without building from source, take heed: Bleeding-edge NCO features 
are available before official releases. 
New features are built into pre-release .deb packages as the 
features are introduced.  Once an (arbritrary) number of new features
seems stable, we simply announce that the current pre-release code is
the official (stable) release code. For example, .deb packages with
the 4.2.5 release name have been at the expected location

http://nco.sf.net/src/nco_4.2.5-1_amd64.deb

for many months, but the NCO homepage for binary package download
only points to this location once the release is official. Until then,
they are rebuilt and replaced as each new feature is added. When the
release is official, the package is frozen and a package with the next 
version number 

http://nco.sf.net/src/nco_4.2.5-1_amd64.deb

is created. This is announced once it stabilizes a few months later. 
And so on. With judicious use of dpkg, Debian/Ubuntu users can easily 
track NCO feature development more closely than official releases.
Just remove your existing NCO package and replace it with the latest
(unannounced) package that has the feature you want with, e.g., 

wget http://nco.sf.net/src/nco_4.2.5-1_amd64.deb .
sudo dpkg --remove nco
sudo dpkg --install nco_4.2.5-1_*.deb

or, for AIX,

wget http://nco.sf.net/src/nco-4.2.5.aix53.tar.gz .
tar xvzf nco-4.2.5.aix53.tar.gz .

Caveats: 
1. .debs for amd64 (my development machine) are available early. Savvy
users can build the binary .deb for any architecture using the other
files (.changes, .diff.gz, .dsc, and .tar.gz) found there.
2. We encourage users to try pre-release packages because getting
feedback on features from those who most need the features is
immensely valuable to stabilizing the code for the final release.  
However, pre-release packages are not for the meek. 
A pre-release NCO may crush one's puppy or eat one's cat. 
That's probably the worst that could happen.
3. These .debs do not yet support netCDF4 (they soon will).
For netCDF4 support, one must still build NCO from source. 
Or one can use NCO with netCDF4 in RPMs (e.g., from Fedora) though
those are not built until after official NCO releases so one cannot
preview features with pre-built RPMs.   
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End Debian/Ubuntu pre-release explanation
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