It's where the cylinder slips and rotates on the yoke tube or it may be called the crane. It seems that eIther that tube or the internal of the cylinder are worn and it causes some play that seems to allow the front of the cylinder to go side to side more than normal like the two pieces don't fit really tight. The tube may officially be called the crane. It's the tube on the yoke that the extractor goes through and the cylinder revolves on. Like steelslaver described. The yoke is numbered to the gun so I don't want to replace it.