Definitely an iron sabot.
I agree with Col. Bartleson and CWArtillery, the "hole" is located too close to the shell's bottom to be able to connect into its powder-cavity. Based on size-comparison with the 1/2"-long sabot, the hole is only 3/8" above the iron main body's end. That puts it well-below the bottom of the shell's powder-cavity. For that reason, it can't be either a "sideloader" hole or a deactivation-drilling hole. See the photos at the end of this post, which show this type of shell's base is 1-&-1/2"-thick below the powder-cavity. (It is approximately the same thickness whether it is US-made or CS-made.) A side-hole at that location would go into the shell's thick solid base, not into the shell's cavity.
Also, the "side-hole" on that shell is too small to fit case-shot balls into.
I believe Divedigger is on the right track about it. For this type of shell, the ironcasting mold's filler hole was located at the outer edge of the shell's main iron body, very slightly above the sabot. A "casting irregularity" tended to occur at that area. I've seen such irregularities there many times on this type of shell.
Note: I modified this post to correct my mis-typing of Col. Bartleson's name.
Regards,
Pete