If you have access to a copy of Charles W. Pate's excellent book The Colt Model 1860 Army Revolver (2018), on p.328, in the section wherein he discusses the model differences, there is a photo of a Fourth Model 3-screw CFS, and your SN 96310 fits within the lowest/highest numbers observed, which is quite a large range. Pate also mentions, as Scooby points out, that Colt never wasted a part for the assembly line.
Nathan L. Swayze's '51 Colt Navies (1967) also states the same, and has photos of, similar 1851 Navy revolvers in the chapter concerning 4-screw CFS revolvers.
I might add, as Mazo's revolver's SN shows, this did not just happen after the ACW, but during, and even prior to it as concerns the Navies.
Regards,
Jim
