For the knife....
I think the monkey on knife is Vali.
Vali has the power in combat more than Hanuman.
Vali is son of Indra (the leader of Gods), Hanuman is son of Vayu (God of wind).
Vali has special power from Indra. when he fight, rival's power would be reduced by half.
He is the best warrior and has victory over Ravana several times.
He had a prominent role in the story is that be beat Pali.
I think the owner might want to put that faith into the knife with the power to overcome evil.

Other reasons, on the characteristics of mask, Vali is the King. He has a crown on his head.
But Hanuman is the warlord. He hasn’t crown and has a special fang in the center of his mouth. The mask of Hanuman must be open mouth.

This is my opinion.

Your friend,
Mor.