Tiger Still on 8A series


I've expressed before an interest in Apple build number conventions.

The latest Mac OS X 1.4 "Tiger" build number is apparently 8A425.

What is unusual is that they are still on the "A" series unlike previous releases:

Interestingly, the middle letter has consistently been lower each major release.

I'd still love to know what triggers a change in the middle letter (and why that trigger never happened in Tiger).