The Rebs put a limited quantity of "zinc fuzes" in their rifled shells and round cannon balls. Your Vicksburg relic could be one of those whose fuze was apparently filed down or otherwise messed with after the war. It appears to be surrounded with white lead which is something the arsenals occasionally did to prevent early explosions from flame passing around the fuze. The two shells below are candidates for having those zinc fuses although it appears to be some lead alloy. Maybe the zinc was mixed with lead and a little tin to harden it.
The two page letter below is filled with important information from the Augusta shell trials in January 1864. Written by Col. Oladowski, ordnance chief of the Army of Tennessee, he had personally attended the Augusta trials where they fired 20 ten pounder Parrott shells equipped with Girardey percussion fuzes. Seventeen of them exploded properly. Most were set in copper fuze plugs. Five were set in zinc fuzes like the example below. Of the three that failed, two tumbled badly because their sabots did not take the rifling, one exploded as it left the gun. In another report from Augusta, they tested many of Col. Broun's concussion fuzes and fewer than 50% exploded properly. At the time, the Broun fuze was being mass produced in Richmond for the forthcoming spring campaign. Olandowski asks for the Girardey fuze the become the standard regulation fuze for his artillery. In Richmond, it was ignored in favor of the inferior Broun fuze until sometime around May 1864 when largescale use of the Girardey began.
Another important clue in this missive is the description of a proper copper sabot with three holes in the top. This strongly suggest that Read shells were still regulation items at the end of January 1864. Another letter from late March 1864 talks about the single wide band on their 3 inch ammunition. We're getting closer to pinning down the timing of the switch from Reads to 3 inch Brouns. And a final noteworthy item is the letter's mention of the 3 inch Reads they tested with holes in the sabots to allow flame to pass to the fuze. Mallet advises against using the copper sabots with flame grooves. These are positively the mysterious Reads with four flame grooves. Even though an intact example has recently been dug in Richmond, this was a Deep South shell I believe made by Augusta at the end of 1863. I suspect Richmond's find was at a test site. Asa Snyder made all of those well known Reads with three flame grooves in Richmond at the end of 1862. Col. Mallet included a sketch of this Augusta sabot in his notes from the trials showing the four separate flame grooves. Enough for now.