The "garland" is actually a thin piece of donut shape brass that is about 3/4" WIDE --- not thick. The photos of your trumpet show it does not have a garland around the bell but that is not unusual. Many were made without them. The mouth piece is correct for the time period through the later Indian Wars.
While the
Quartermaster Manual, May 1865, Camp and Garrison Equipage stated regulation trumpets being about 14 1/4" high (probably just the trumpet and not including the mouth piece) and with the "bowl" (bell) about 5 1/2" in diameter and made of brass, in 1879 new regulations came out for a two coil brass cavalry trumpet 16" overall (including the mouth piece) and with a bell diameter of 4 1/2."
The Nebraska State Historical Society has an "Austrian-made brass bugle, used by James W. Thomas, Co. H, 2nd Nebraska Cavalry that looks similar to yours but not exactly alike. Nebraska became a state in 1867 so this isn't much help. Trumpets and bugles can be confusing as Civil War style instruments continued to be made decades after the end of the Civil War. These were mostly made of brass however instead of copper which was prevalent during the ACW.
To complicate matters even more is the fact that state regiments provided their units with trumpets and bugles that were anything but "regulation." Instrument makers were located in a number of northern states. Some were imported from Europe. All this makes it difficult to date a bugle or trumpet though copper single coil bugles (some with a brass garland) are typical of the Mexican War and the ACW.
Then too, old veterans and members of the GAR who had been buglers during the war later provided themselves with post war instruments to play at reunions. Your trumpet however shows the kind of hard use that certainly speaks volumes about cavalry during the Civil War -- cavalry trumpets probably show the most dents and damage. Being dropped from horseback would be unforgiving on a soft copper or brass instrument.
I'll post a page showing the 1879 regulation trumpet that adds the confusion when it comes to dating a trumpet. What it does show is a similarity to the regulations that came out in 1865.
Do you have any other items like a discharge paper or photo of your ancestor in uniform?