It all my years of searching, I've never found something quite this old! And what a pleasure it is to have such an item presented here. This is a 1904 US Army brown canvas fatigue shirt, known formally as Coat, Canvas, Fatigue, specification No. 612. These were adopted by the army in 1903, and would be used until replaced with blue denim around 1918.
Of most interest with this item is the stamp inside. Western Costume are still one of the largest suppliers of costumes to the entertainment industry. Founded in 1912, they would buy up large quantities of surplus uniforms, and these would be hired out to studios for use in productions. Its only really in the last 20-30 years that films have started to reproduce garments, previously they would use originals. There are incredible stories of their warehouses containing rack upon rack of WW1 and WW2 pieces that today would be worth a fortune! Many of the early prewar denim army pieces today survive due to Western Costume, and the stamp can be found on a lot of them!
Condition
This is in stunning vintage condition! Look carefully at the split pin donut button with a slight bit of rusting, these being very common on late 19th/early 20th century denim pieces! The canvas tag has faded, but you can see where it would have been on the inside of the button placket.