St George is correct!
The base of the sight is contoured to fit the TD Springfield's
The percussion Sharps barrel is a slightly larger diameter, leaving something of a space between the sight base and barrel.
The Springfield sight is held in place by two screws one in the front, under the sight ladder, and one at the table you see at the rear. Both need to be removed to put the correct rear sight back on.
This will probably leave you with a screw hole in the barrel to the rear of the proper sight.
Fill this hole with a screw and clean up anything that sticks out of the hole.
Hope this helps?
My best,
Blair