Your best bet in actually dating those decorations is going to be a period catalog from a supplier to the Saddlery trade.
If it appears on horse equipments - then most likely - in some form - it'll also appear on that worn by the Cowboy - and decorations of that type have a long history going back across the pond.
The older rigs I've seen or owned were from the late 1880's and on, and were fairly simply made - yet 'visually arresting' - as a couple of the ones seen in 'Packing Iron'.
Probably due to the fact that in large part - the 'true' gun rig - the one with a cartridge belt as opposed to a waist belt - was in wide use and highly visible, and offered more 'surface' for decoration.
The American Cowboy liked a bit of ostentation, from time to time - and while he never had much money - what he had - he'd spend on things that could be 'seen' and that struck his fancy.
No Shrinking Violet, he...
I've seen entire rigs where spots and rivets were what held the whole thing together - but those were 'Bunkhouse-made' , and the quality varied with the abilities of the Cowboy and those things that were available to him at that moment.
Good luck in finding dated documentation, and when you do - it'd be good to share your sources with everyone.
Vaya,
Scouts Out!