I see this thread has not previously been resolved.

Definitions of the many terms used in aero engine testing started to appear during the late 1920s and were later included in various AID and Civil Airworthiness Design Leaflets.

National Archives documents AVIA 2/2123 and AVIA 2129 gives background information as to discussions held with other National agencies.

During the 1930s the following definitions were included in various editions of AP 1208 and numerous AID leaflets concerned with engine testing:

International, Special* or Rated Power is the corrected brake hp (established on a rating test) developed at full throttle when the engine is running at the International, Special or Normal rpm and under the standard atmospheric conditions of the rated altitude. A tolerance up to 5 per cent is permitted for the minimum accepted power of Series engines.

Rated Altitude is “full-throttle power” and quoted to the nearest lower multiple of 5 hp for engines above 150 hp and to the nearest lower multiple of 2 hp or 5 hp for engines below 150 hp.

International, Special* or Normal RPM is the maximum crankshaft speed, at which the engine may be run for long periods, as declared by the maker.

*When referring to Normal category engines, the terms “International power” and “International RPM” are used, and when referring to Special category engines the word “Special” is used in place of “International”.