NTP uses A.B.C - style release numbers.

At the moment:

 A is 4, for ntp V4.
 B is the major release number.
 C is the minor release number.  Even numbers are 'stable' releases and
 odd numbers are "development" releases.

Following the release number may be the letter 'p' followed by a number.
This indicates a point (or patch) release.

Release candidates have -RC in the release number.

Here are some recent versions numbers as an example:

 4.2.2		A production release (from the ntp-stable repository)
 4.2.2p2	A production release (from the ntp-stable repository)
 4.2.3p12	A development release
 4.2.3p15-rc1	A release candidate for 4.2.4

Note that after the ntp-dev repo produces a production release it will
be copied into the ntp-stable and the cycle will repeat.

Feel free to suggest improvements...