Alan
60087D is A139 4K-AZ125 Silk Way Helicopters c/n 31506, if so I would have thought 4K-AZ126 would be 60087E (if it exists) as 4K-AZ127 c/n 31510 is known to be 60087F.
600879 is therefore most likely to be A139 4K-AZ121 c/n 31495 which is our missing one if all are in a fixed hex/reg sequence
X