Blood Door Theory
The blood door is a very interesting thing in my opinion. It has been theorized alot but there's still no actual anwsers to it, so here's my theory about it!
So starting off you can see alot of blood on the floor in this picture, that's why it's called the blood door.
You can see that 3 = C is on the wall, for the puzzle in Submachine 2, but it serves no purpose here. You can also get a cogwheel which is in the blood.