1891 Argentine should be a flat bottom receiver.
Use your calipers as accurate jaws to hold your mount on the top of the receiver to get center position after you remove the loading hump.
This link should help. I use this method on Mexican, Argentine, Spanish, Turkish, and others.
Failure is part of success!
This year will go down in history. For the first time, a civilized nation has full gun registration. Our streets will be safer, our police more efficient, and the world will follow our lead into the future!" -Adolf Hitler,1935"