wrench

Two possible solutions - fill in R4/c4, then it´s completely logical (although very difficult).

Even with the given hint, I don't thing it's logical. More guessing needed.

Hard, bur guessable!

No chance to solve it.

The "Wrench" is top left to bottom right, in each of the opposite corners there are 2 allen wrenches and then there is a "T" in the top middle and a hollow square in the bottom middle, both are slightly off centre. I Hope that helps anyone still struggling without giving away every position.