2 3-4 almost never. If the gun has been in the us long? If so most likely answer is somebody eased the chambers from 2.5 to 2.75. Measure the wall thickness. If greater than .100 you should be ok with same pressure loads. If under .080 I would not shoot it. Between you must decide.

2. 1/2 was standard for the period. Even longer shells often just were loaded with more, better wadding. Nothing was loaded to what our factory 2 3/4 factory hunting loads are today. Thats a complete non starter. Plus at 130+ years caution is your first responsibility to the gun, yourself and others.