This engine was a Briggs Sonoduct, first built in 1960. Contrary to original poster, design was meant to lower center of gravity, not necessarily to improve serviceability. In fact engine was more difficult to service ignition, as engine would have to be removed from mower for any points, condenser, or coil service.
Would have been a better design 20 years later with electronic (Magnetron) ignition. In fact, descendants of this design are the "half crank" 2 stroke engines commonly seen on low price weedeaters, blowers, etc.