There is a Metro III heavy in Oz that this has happened to before.
It occured around 1992/3 (approx) when the aircraft was with American Eagle or whatever they were called then.
After repair the aircraft went through a refurb with Merlin Express/Fairchild and was sold to an Oz operator. The aircraft was delivered to Oz (late 94)with the photos of the damage - big (BIG) hole under and behind co-pilot's side window. Certainly made everyone do a double-take at the time.
Cause was oxygen related as Richo has described.
Wouldn't it be interesting if it was the same airframe?
Edit the above: Clearly if its a M23 in BN, its not the same airframe! D'oh!