Bunn Leonard as you stated the button was a 1868 re-release of the period button made from the same dies. Since the northern makers could not ship them "legally" south after the Civil War started after large orders had been placed. Some were known to be smuggled South and there were probably bins of Period buttons in warehouses that were embargoed from going to Dixie. Many of the mint non dug Southern buttons being sold today for very high prices are likely from out of storage of that scenario. Possibilities are best that yours was made as a period button and lost during the war. Your reference source states a re-issue of that button in style and back-mark. Probably the order was not very large as Mitchell& Tyler was an outfitter not a manufacturer and Tyler had passed away I think at that time, no need to pay for a new set of dies when you might already have the buttons in storage or the order was too small to merit re-tooling. There was no major military manning up in 1868 in Virginia to require a large button order as was the case of 1860 and 1861 when militia units began to form and enlarge due to the secession issue.
There are buttons Called Civil War that were never issued to troops and no dug examples are known(many Van Wart Examples)but are still considered period buttons, coming to light out of warehouses off shore.
If I were you, I'd call it Period until someone had definitive proof that said otherwise. Your source says no known buttons marked C. D. Carr of Charleston, SC dug this with a palmetto Guard (SC17) and have seen others.
[attachment 13685 c.d.carrA.jpg]
Many Wendlinger Va staff buttons have been dug on Va battlefields but are considered post war although Wendlinger was another outfitter in Richmond,Va during the war along with Mitchell & Tyler. Just my opinion
5forksva