Thanks for posting the pics of your new acquisition.       Cyl on the barrel means it is a cylinder bored barrel without a choke constriction.  The 2 3/4 " shells will fit your gun but don't beat it up with a lot of high brass, high pressure loads.  Just the regular pressure stuff will work fine and keep the gun functioning better and longer.  It looks like you might have a hairline crack on the right side of the pistol grip next to the receiver.  Keep an eye on that to make sure it doesn't get worse.  Some can be repaired - some not so much.  Check with a shotgun specialist to determine what the status of it is. 

I think (operative word, there, so don't bet the mortgage on it) that the orginal 1897 riot guns and trench brooms were equipped with steel buttplates that fit that profile.  Don't quote me on that, I could be as full of s**t as a Christmas goose.

The 2 3/4 " shells will fit your gun but don't beat it up with a lot of high brass, high pressure loads.  Just the regular pressure stuff will work fine and keep the gun functioning better and longer.   

Thinking of target and upland game loads as lower pressure loads is a common misconception.  Pressure in modern shotshells run close to the same in both light and heavy payload cartridges. This results from the use of "fast" shotshell powders in light payload shells and slower burning propellants in heavy payload shotshells.

I think (operative word, there, so don't bet the mortgage on it) that the orginal 1897 riot guns and trench brooms were equipped with steel buttplates that fit that profile.  Don't quote me on that, I could be as full of s**t as a Christmas goose.

It certainly looks like that could have been the purpose.  There are no (apparent) screw holes there, but the wood notch is such that screws wouldn't necessarily be required.  And there are so few true original guns around, that I couldn't find any pictures showing a steel buttplate.  Doesn't mean it didn't happen; it just means I didn't see one.