At the end of the 1.4 series, major steps where taken.
A whole new version (1.5) and a development plan was formed.
According to tcl-commands.doc, the version numbering is as follows.
The above, in the way we normaly see it is.numversion
Value: the current numeric bot version (for example: "1010201").
Numerical version is in the format of "MNNRRPP", where:
M major release number
NN minor release number
RR sub-release number
PP patch level for that sub-release
N.MM.RR+PP
However, the +PP isn't normaly there, and will only apply to the likes of the IPv6 patch, or inbetween versions.
Only when the "Minor release number" is ODD, is it a development release.
As such, 1.6 is even, and is considered stable. 1.5 and 1.7 are odd, so where/are beta/alpha quality.
All 1.6.x bots are considered stable, unless a specific message says so (IE, the issue with 1.6.11 cause major socks problems).