Not a DC 8 expert but the production listing shows s/n 46147 reg N811CK as a DC8-63AF and s/n 46151 reg N815CK also as DC8-63AF.
There is often confusion with "common" or "popular" type designators but per the FAA Type Data Sheet the -63AF means All Freighter.
This is the Douglas designator for a production built freighter aircraft with no windows or passenger services installed.
With factory installed door and floor these were inevitable lighter than the 60 series passenger aircraft that were converted by third party STC's.
Original MTOW was 161,025 Kg being the heaviest of the DC8-60's.
Of course payload will be reduced with the Stage 3 mods. <img src="wink.gif" border="0"> <img src="wink.gif" border="0">