Question solved! Then come to think of it, I have actually seen one of those do some approaches somewhere at a US airfield years ago.
Below a list of the various C-18s (from:
http://en.wikipedia.org/wiki/C-137_Stratoliner )
C-18
An EC-18B Advanced Range Instrumentation Aircraft (ARIA) takes off on its first flight at Wright-Patterson AFB, Ohio, following its conversion from a Boeing 707-320.
The C-18 is the US military designation for the conversions of the 707-320B series.
C-18A
Eight second-hand (former American Airlines) 707-323Cs bought as crew trainers for the EC-18Bs, four later converted to EC-18B, two converted to EC-18D, one to C-18B; one was not taken into service and was used for spares.
C-18B
One C-18A modified with instrumentation and equipment to support the Military Strategic and Tactical Relay System (MILSTAR).[2]
EC-18B
Four C-18As modified alongside examples of the C-135 for Advanced Range Instrumentation Aircraft (ARIA) missions in support of the Apollo space program.[2]
EC-18C
Original designation for two prototype J-STAR aircraft, later redesignated E-8A.
EC-18D
Two C-18As modified as a Cruise Missile Mission Control Aircraft (CMMCA).[2]
TC-18E
Two second-hand (former Trans World Airlines) 707-331 aircraft modified for E-3 pilot and crew training.[2]
TC-18F
Two second-hand (former TAP Portugal) 707-382 aircraft modified for E-6 pilot training.[2]
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