I have just stripped down the GSG5 that a club member said there were pieces of metal falling out of!
There is a cover on top of the breech block. This was fractured in three places and one of the securing screws has come out. The whole of one front corner was broken off and what appears to be guide rails on each side were broken away.
The rear of the breech block is heavily worn, indented and burred up where it works against the hammer to cock it on rearward movement. That appears to be impact damage from the bolt hitting the hammer.
There is a cross bolt described in the parts list as a locating pin. This appears to work as a bolt stop and it is impact damaged by the bolt hitting it with heavy indentation and burrs set up.
Points to look at when buying or stripping for cleaning.