I don't think there is premature unlocking, as that is about impossible in a gas gun where the bullet has to reach the gas port to start the unlocking process. I don't think the gun was ever locked, and that somehow the hammer reached the firing pin with the bolt partly open.
I see what you mean about the line, though. It looks like what you might see in a dropping barrel pistol, but the FN-49 bolt doesn't drop at the front. The only way I can see that line happening at the time of firing is if the bolt was actually blown upward or to the side, leaving that mark as the firing pin scraped the cartridge case. What happened to the bolt and bolt carrier? Did they blow out of the gun?
Also can you tell where the extractor was? Those look like extractor scrape marks on the (viewer's) right side of the picture. The extractor position would let us orient the case the way it was in the gun.
A thought, though. One thing that could stop the bolt carrier and the bolt would be the gas piston if it stuck out part way. Can you check to see if it might be bent or have a burr or something that could do that?