Gordy. You're right I'm referring to 14 CFR 61.56 which requires the flight review and logbook endorsement to be carried out by 'an authorised instructor'. That person is presumably usually going to be a Certified Flight Instructor but may also be 'an examiner, an approved pilot check airman, or a U.S. Armed Force' again according to the FAA Chief Counsel.
Worth noting also that the 24 month time limit is very precise with no leeway for extensions, defined as: "... the beginning of the 24th calendar month before the month in which that pilot acts as pilot in command'.