Looks like an early 'We are heroes' photo. The fact it is 'non-regulation uniform' suggests they have not been issued the 'standard' uniform. No headdress and no equipment at all. Here is my usual 'tidy up:
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It definitely is an Ambrotype, as there is a long crack across it (mirror-image, on glass). It was VERY spotted too and was peeling in several places. While working on it, at a much higher resolution, I doubt if it is a uniform at all with the exception of the left-hand man's jacket, but even then ... The cut and 'lay' of the material does not look like it. Bear in mind that this was a well-tinted photo too, which covers some detail of these usually sharp images.
The old question about 'studio props' is also raised, looking at the pistols produced, although these MAY be personal purchase. The Colt Army 1860 is a questionable one as how many in the Confederate army were issued one or could buy one? Or was this taken AFTER the war - the old soldier on the left?
The pose is also 'unusual, as they are holding the pistols in their right hand. Since the photographer knows it is a mirror image, he usually gets them to swap hands and rearranges any equipment to make it look right.