Early BA 747-136's had P & W JT9D-3A engines. After the JT9D-7 went into service P&W offered a conversion kit to upgrade the -3A's to -7CN's (CN for converted) which retained a lot of the external bits in the same locations as the -3A but installed -7 internal bits especially the combustion and turbine sections and upped the thrust. BA used to paint -7A or -7CN on the engine exterior as the component location differences were significant and it helped to know what was under the cowls before starting work.