It is your bullet #17 (bullet #35 in the T&T "Handbook"), a Confederate .44 caliber "Richmond Laboratory" bullet, except that this one has been fired. Firing it through the .44 revolver squeezed its original .455"-diameter down to .438-inch.
For anybody here who doesn't already know... a firearm's "caliber" (such as .22 or .38 or .44) is the diameter-measurement of its bore (the tunnel through the gunbarrel) -- not the diameter of the UNFIRED bullet for that firearm.