I will tell you the situation, and you tell me how to classify it. :-)

The 5.005 series was a significant change from 5.004 because a lot of work was needed to make Windows one of the core platforms. Therefore the following releases happened in order:

5.004_04
5.005_00
5.005_01 (very shortly after IIRC)
5.005_02
5.005_03
5.004_05 (backported bug fixes for people who didn't switch)
5.6.0 (Note new numbering scheme)

If history repeats we would see more upgrades to 5.6.x, followed in due time by backported bug-fixes labelled 5.005_04.

So the last released version called stable before 5.6.0 was indeed 5.004_05.

Cheers,
Ben