Judging by the commit messages, 1.3.3 is presumably the sources which were
frozen at 1.3 as amended by 1.3.1 and 1.3.2 plus some new fixes.  1.4 is what
the mainline will become when it is formally released.  Thus lettered versions
(1.3I) represent 1.4 minus some unknown quantity of work, not 1.3.n (for any
particular value of n) plus some work.

I seem to vaguely recall suggestions that SMP would be a good candidate for
a major enough change to warrant calling it 2.0.

Of course, I'm not a member of core, and don't even play one on TV.  I also
don't have every current-users message for the past 8 years memorized, so
I could be way, WAY off.