Re: picture of the 3rd Indiana Cavalry from Petersburg
All points raised are certainly plausible. The possibility of the jackets being some kind of state issued jackets is possible but not likely in 1864. Perhaps they field modified their regulation issued jackets based on their favored early war state jackets?
Just to stir the pot a bit...and sorry I am not good with links or attachments but if you do a search of the book "A Damned Iowa Greyhound" about William Henry Harrison Clayton from the 19th Iowa Infantry look at the picture on the cover. He is wearing a regulation issued frock coat and it has a low collar too. I can't remember now but I believe that picture is after his release from Camp Ford, Texas in 1864. Maybe that is a field modification or some kind of private purchase.
Honoring: Pvt. Albert R. Cumpston (Company B, 12th Illinois Vol. Infantry- wounded February 15, 1862)
Pvt. William H. Cumpston (Company B, 12th Illinois Vol. Infantry-Killed February 15, 1862 Ft. Donelson)
Pvt. Simon Sams (Co. C, 18th Iowa Infantry-Killed January 8, 1863 Springfield, MO)
"...in the hottest of the fight, some of the rebs yelled out...them must be Iowa boys". Charles O. Musser 29th Iowa Infantry