Option 1 (the corselet is a complete fake): Someone in the early 20th century cut out the scales from a brass sheet and asembled them into a corselet.
Option 2 (the scales are genuine but not the corselet). Some loose scales from what was originally a horse trapper were illegally looted from somewhere and assembled into a corselet.

The neck and shoulder pieces are a modern fabrication. If they are made from the same metal as the scales then we know the whole thing is a fake.

The last line on the ROM poster which Greg provided above makes it clear these scales were more recently cut from a patinated sheet. That is, the edges are bright, but the face is green. Points to modern fake, IMHO.