I have Browning's isometric cutaway drawings on the first page of his patent application. It appears that the shell carrier pivots from its rear on its carrier pin. Front of carrier secures breech bolt by mating to a projection under the front of the bolt. So instead of the breech bolt locking itself to the receiver or to a barrel extension, it locks to the carrier that locks to . . . what?

Pivoting shell carrier is held in position by the carrier pin (.250-inch pin??). The pin doesn't appear to be sturdy enough to be the single part that allows carrier to lock breech bolt while withstanding the cumulative stress of having 500k shot shells run through its 1897 action.

The carrier has several projecting portions of it that do several things. I do not understand what are the functions of some of these lugs and projections. I would think that it would be a good idea to have the back or the bottom rear of the carrier abut the rear of the 1897's receiver. What I mean is that something at the rear or bottom of the carrier behind its carrier pin should mate to the receiver to accept the stress of firing. And the carrier pin should serve as a pivot pin, perhaps accepting a small amount of firing stress. But I cannot confirm that the rear of the receiver has mass necessary to support the carrier during firing the shotgun.

Not having disassembled a Winchester Model 1897 pump shotgun, I request help in understanding how the mechanism locks breech bolt for firing.

This might help

https://www.youtube.com/watch?v=Abf8PS-sMHI

Yes the carrier does lock into the rear of the frame and the breech bolt is held by the carrier; that pin is just a pivot pin. The system is quite strong.
The lugs on the carrier are cams which the slide arm uses to move the carrier up and down; that is the weak link as these cams wear and then the shotgun does not function well or at all.

Many thanks. Since I didn't know the difficulty of finding the answer, I created threads on three "gunsmithing" subforums. And I got the answer to my question on all of them. Despite finding the answer being relatively easy to find, I appreciate your taking the time to take a look at your 1897 for me.