Asket said

What original front slings were used? is the original sling in the photo?

  

I have never seen a factory installed sling on a standard (Field grade) Model 1897 Shotgun.  The WW I and WW II Riot and Trench Guns were configured for the same sling used by the U.S. Military on their rifles.

Asket said
I was studying the sale of spare parts for Winchester 1897 and I had a question:

1897 Type C with three screw has a “blocker” on the magazine tube.

What function does it perform?

Type C does not have a action slide screw cap!

https://photos.app.goo.gl/Pz2Q6xKMbFLDt5fA8  

The piece you refer to is found on all Model 1897 Take Down magazine tubes.  It prevents over travel of the slide assembly when you slide it forward to clear the magazine tube from the receiver frame.  It is held to the barrel with two countersunk screws on the “C”, “D”, and early “E” series guns.  On the late production Model 97 shotguns, it is an integral piece on the magazine tube, and it has a different shape.

Asket said
Friends, tell me, plz:

What is the distance from the cut of the barrel to the front sights?

  

The precise distance from the muzzle face to the dead center of the sight bead is .500″