I believe the -351Bs with the cargo doors were delivered about a year or so before the C models were available. From what I remember, NW
took these planes in 1962, and the C model did not come off the production line until 63 or 64.
The combi NW
-351Bs lacked the "hat rack" exits just aft of the wing found on the true C models. Also, in the original NW
configuration, the combi -351Bs had a compartment permanently walled off for cargo on the left forward side of the cabin from the entry door aft to about the wing leading edge, with normal rows of FC seats on the RH
side of the forward cabin opposite that compartment. Later true C models could be configured as combis, but not in the same manner -- there was a normal style, movable bulkhead that separated cargo from passenger in the case of the C model operating as a combi.
[Edited 2012-02-05 18:41:20]