How to unlock W'?

[Strontium 90]

I've been playing for a while and can't seem to figure out the unlock condition for W'. Can i get some help?
Edit: I found out it is unlocked by perfecting 3 levels of P7x7

[Perseus]

Are you asking because you don't even have the unlock conditions yet, but you looked at the achievements?
It's probably related to completing some of Wall's larger levels, but if there's any other condition it might be related to Partition as well? Maybe?

[Strontium 90]

Originally posted by Perseus:

Are you asking because you don't even have the unlock conditions yet, but you looked at the achievements?
It's probably related to completing some of Wall's larger levels, but if there's any other condition it might be related to Partition as well? Maybe?

I know it exists mainly due to the unlock condition for #', but i've unlocked W5-8 and none of them say anything about W' and it is not visible in the ? section even as a lock icon