The Merlin was cleared for +12lb on Take off and that could be used for "short duration" in an emergency. It was so limited, that it was not cleared for even ONE full minute much less FIVE.
It clearly states that the engine is highly overloaded when using +12lb boost. Furthermore, using it for short duration outside of take off, immediately deadlines the engine until it is inspected by a mechanic and cleared for re-entry into service.
How do those very clear instructions get translated into "could use +12lbs continuously"?
The endurance testing quoted in this thread is extremely limited for an endurance test. Most engines are ran continuously for far longer time periods at the emergency conditions during endurance trails with resulting tolerance wear. The Merlin was run for only 5 minutes at a time with a 20 minute rest period between. In that context, the Merlin endurance trials at +12lbs were not successful and the results are far from the "idea" that the Merlin was cleared to run +12lb continuously.