Here's a suggestion that could bear on your question. A metal press used to make the belt buckles you mentioned probably didn't remain idle just because the company wearing the original set of buckles had been dispatched to Virginia. The press operator could have been pursuaded to make more for other men enrolled in the Confederate military. Unless the original had been patented, there's no reason to assume that only a member of the Vicksburg company mentioned could have worn these buckles.