Large opening in the throat of the sabre.
 

In the case of a sabre guard attached to the handle shaft, a large hole is encountered on the side of the blade.
Attachment 238789
Attachment 238790
Please explain what was the purpose of making the guard in this way?
It seems that a guard that does not fit tightly to the blade during use will loosen quite quickly and will protect the hand poorly.
Some of the pictures show filling this hole with some kind of glue or something similar. This is unlikely to be a permanent connection.

I wonder if that recess is for the upper locket of the scabbard to fit into. In the second picture, it appears that the crossguard is anchored to the pin securing the grips.

In this photo, you can see that the crossguard was additionally attached to the pin. My guess is that it has just come loose.
Photo of a Cossack sabre from excavations on the battlefield.
Attachment 238792

Yeah, I think Lee is right. It basically makes the crossguard function like a rain guard.

Also remember that guards and blades would often be made in different shops and assembled in a third. This is a sidearm that will be replaced if it ever receives significant battle damage.

Robert