The visible pattern on the Bridgeport knee is flaking, not scraping. It is done on the finished surface to aid lubrication and reduce stick/slip. The actual scraping can be seen beneath the flaking, it appears as wider, straight marking.

Scraping is an iterative process on machine tools to produce precision surfaces. Flaking is a secondary operation.

The finish on the bolt in the OP appears to have been done as RM Bill described, for purposes unknown.

Jim H.