With the SKS and FN-49, as well as the FN-49's daughter, the FN-FAL, the rear portion of the bolt tilts down to engage the locking shoulders recessed in the receiver. The bolt carrier has cams machined into it to lift the tail end of the bolt out of the locking shoulders and allow it to move backwards during the extraction/feeding cycle. That's also why the bolt face is at an angle to the bolt's centerline axis, so it's square with the bore axis when the bolt is tilted down at the rear during full lockup.

Huh. How about that. Sorry for cluttering your thread.