Other than Alton, the men men show no company designation in their CMSRs.
I checked the records of Ponds and Purcell.and found the only record of them was that they were surrendered at Vicksburg. No enlistment data and no muster roll. Nothing to indicate their post seige service.
Most, if not all, of the rest probably show the same.
Is it possible that they were recent conscripts and never fully assigned to any particular company?
I wish you the best in your research.