This diagram is for the Farm Pump Engine and all its variants having a steel battery box.  Early pump engines did not have a place to mount the battery box on the engine.  They used the standard wood battery box but the internal wiring hookup was the same.  Tag 5 refers to the insulated binding post on the cam rocker.  Tag 4 refers to the grounding post, this was either located behind the battery box right on the crankcase or on the steel post holding the detent lever adjustment screw.  Tag 3 of course was the sparkplug.