How did Power Guard behave pre-Ascension?

Did it block only physical damage, touch damage, or all damage?

Post Chaos it blocked all damage, magic and weapon.

Post Chaos it blocked all damage, magic and weapon.

No, it only blocked damage that was considered physical. There are special attacks monsters use that are still considered physical attacks. One such example would be the pseudo body slam that the various Guardians in the ToT used. Even though the attack could be used at a range, it was still a physical attack, and could thus be blocked by Power Guard. This also applies to some of the 1/1 attacks, like Zakum's, which Power Guard could block, but other 1/1 attacks from bosses like Griffey and Manon were not blocked because they were considered magical.

Basically, Power Guard was upgraded to work on any attack that was deemed physical by the programming, but not magical attacks, while now it works against all forms of damage, including magical.