sfs201 is the part number for a normal 5/8ths deep-well 3/8 drive socket. As you can see someone cut a slot in it. likely they cut the slot for a specific job. think of an o2 sensor socket w/ a slot cut in it. however, o2 sensors are 7/8ths or 22mm.
so my best guess is they cut a slot in it to remove a pigtailed sensor, or maybe used it in conjunction w/ a valve spring compressor. the slot allows them to grab the keepers w/ a magnet while the spring is compressed. last thing i can think of is maybe to remove a diesel injector--but most diesel injectors i ever seen are bigger than 5/8ths.