This is an original Aertex British WWII RAF marked "Bush Shirt" made in 1945. As you can see, it was tailored to short sleeves, and the collar was removed. I have no way of knowing for sure when this was done, but it was not done recently, and was done by a professional tailor.
See photos for tag and stamps. All buttons are intact (one replacement), shirt is in great shape with almost no staining or damage. The only issues are both shoulders where there are stitch marks and a couple small holes, and some missing fabric at the end of both epaulets (pictured).
Measurements:
Chest: 41”
Collar: ~15”
Length from collar to bottom: 30”