Winchester's 1897 shotgun appears to have its breech bolt lock into battery by being locked by its cartridge carrier - that is, locking into something that locks onto . . . what? I have not viewed a cutaway of the 97 to identify into/onto what the cartridge carrier locks.

Does the rear of the cartridge carrier lock onto a solid part of the receiver? OR is there some sort of bar or rod or cylindrical piece of steel that might resemble a massively thick screw whose visibility has been masked on left and right sides of receiver?

If I have completely missed how the 97 locks into battery, please describe how it's done.

The rounded rear portion of the cartridge carrier rotates in a matching radiused cut out in the back end of the rec'vr.
The front end of the carrier is what locks the bolt shut by positioning itself up and under the L shaped underside of the breech bolt as it is pulled shut.

When in place, the heavy carrier is then the locking bolt and transmits the force from the breech bolt back to the frame. All very stout and strong designed parts.