The beak of the octopus Eledone cirrhosa is located in the buccal mass directly in the middle of the arms under the head. the beak exists out of very tin chitin and is only about two centimeters wide. This model is still a work in progress, still need to refine the details and coloration. Update: found a scientific paper with the buccal musculature of Octopus bimaculoides. Since the beaks of these two species are nearly the same, I can now extrapolate the buccal musculature of Eledone cirrhosa from it. That will be the next itteration of this model.
A version that shows the muscles with different colors can be found here: https://sketchfab.com/3d-models/eledone-cirrhosa-beak-a5abf07896044b27a7c4fd6005d1c5eb