That "C" Was From A Comparer, Not the Original Copyist

There is a reason they refer the compiled military records at the National Archives as "compiled." The cards that researchers use from these compiled military records are extracts from muster rolls and other records.  As such, they can occasionally contain errors.

These two cards are part of ongoing research into Henry C. Markham, a Civil War veteran. These two cards were created by the same copyist as noted on the bottom. 

The card on the left however contains an annotation in red. Apparently the initial copyist left out Henry Markham's middle initial and the comparer noticed the discrepancy and made the correction.

Even without the notation, the middle initial "C" on the left hand card is different from that on the right hand card, even though the copyist is the same. 

Cards from Compiled Military Service Record of Henry C. Markham,
37th Regiment, Iowa Infantry; National Archives.

Notations by comparers were made in red--at least on this card. The distinction made by the color would have been lost in a black and white photocopy. In this case, research doesn't hinge on the "C" or whether the original copyist or the comparer wrote it.

One of the purposes the cards served was to facilitate the processing of Civil War pension claims. These cards were not created for researchers.

But it is worth remembering that information on these cards is secondary and occasionally prone to error.