This is the Subaru EJ20 engine.

EJ20 began service in 1989 when it replaced the EA series of engines. Its compact layout, multiple configurations and very high power to weight ratio has made it very popular among motor sport enthusiasts and custom car builders. It is a 16 valve horizontal flat four available in single and double overhead cam configurations. It is also available in naturally aspirated or intercooler turbo formats. As of 2018 it is still in production but has begun being phased out by the FB series. 

This particular scan is of a circa 2010 model EJ20 from an Impreza RS.

To the best of our knowledge all key elements were consistent across the full life cycle.