Good saves all around. Thanks for the atta-boy comment and depending on your frag's curvature, it might have been one of the big-boys. Unlike my frags (and 9" ball) yours appears to have come from a cylindrical shell (curved in only one direction) vs a spherical ball. Suggest that you stand it on end and use a pencil to scribe the curvature onto a pc of paper. Then using a protractor, tape measure or pc of string find the radius that would follow that curvature. I'm gonna guess it's around 3" (and thus a 6" diameter shell). Measure the wall thickness too - my spherical frags are 1.75" thick, and that's what made my 9" Dahlgren shell weigh in at 72 lbs (plus 4 lbs of black powder!).
Dan in SJ