Slight mistake. Only one of these 4.5 inch noggins behind the top holes.
House was built around 1994. The wall is the back of the wardrobe in my room and behind it is the wardrobe of another room. The walls don't have any power points on them or perpendicular to them. It seems like the same width of wooden beam that I see on the roof under the tiles.
I found the same type of nogging between two other rooms in a normal wall. That one seems to be 5.5 inches wide. Both are at 120cm above the ground.
It's freaky that the middle of this beam lines up exactly with the top back holes of the safe.