This family of definitely humble airlifter have a curious claim of fame; being the unique airframe design to have flown in five different “engine” configurations:
As the XC-123A the second of the two XCG-20 was fitted experimentally with a pair of Boeing B-47 twin engine pods ( J47-GE-11). Test showed good performances, but the low mounted engines acted like vacuum cleaners producing serious foreign object damage (FOD).
The C-123J was conceived to serve (delivering supply, crews, etc) the Distant Early Warning (DEW) line in Alaska. These Providers featured a pair of wingtip mounted J44 jet engines; the extra thrust was more than welcome when operating from the extreme snow runways under full gross weight situations.
After the successful tests of the mixed-powered YC-123H prototype during the early 1960. The USAF decided to equip some of their plain vanilla C-123B with J85 jet pods.
The engine configuration was less drastic than the C-123J and sure easier to service.
The C-123T was an intent to updated the long in the tooth Provider. Not only the legacy P6W R-2800 were replaced with Allison T56’s, the wing was also “wetted” (turned into a fuel tank), an APU added, etc . The Thai AF was interested, but only this prototype was built (first flight 1980); the airframes available were just too old to be profitable.