Really the only way to decide if a DC-3 is a genuine pre-war civil DC-3 is from the constructors number and an accurate production list like the Air Britain book, the rest are military production C-41/C-53/C-47/C-117/R4Ds..Post-war DC-3Cs were low time military airframes civilianized by Douglas and the 28 DC-3Ds were completed from the undelivered cancelled C-47 production line.
As said already the handful of pre-war genuine DC-3s briefly seen at Croydon came from Swissair, KLM , and a couple on the British register.Transair were a significant Dakota operator at Croydon until it closed and produced their own improved version which , like the Pionair had 4-abreast seating .
Question for PASIR...do you remember any more details about the reported KLM Skymaster visit to Croydon when you worked for them?