It is definitely a civil war yankee 3"-caliber Dyer 2nd-Model shell. The "X" is on the shell's base, not its top. Before it exploded, your 3" Dyer shell's base was covered by a thick cup-sabot made of a solder-like metal. Here are a couple of photos to show you what your shell looked like before it exploded. One shows the 100%-intact shell, the other shows the shell missing its sabot. The third photo shows a sawed-in-half view of your shell's interior, with the antipersonnel balls it contained. (In the 3rd photo, the hole through the shell's right side is a modernday drill-hole made during removal of the shell's powder-charge.)