This surprised me...
The 'Bridge of Lions affair' which was also released theatrically as 'One of our spies is missing' is based on a 1963 novel by Henry Slesar called 'Bridge of Lions'. This is one of the only series story line to be based on a pre-existing book.
Slesar was a prolific writer who was popular with Alfred Hitchcock and has written several teleplays for the 'Alfred Hitchcock Presents' based on his own short stories. He has also written for Twilight Zone and Batman among others.
He incidentally won the Edgar award for the best first novel in 1960 for The Grey Flannel Shroud.
'Bridge of Lions' a novel by Henry Slesar and the basis of the Man from U.N.C.L.E episode of the same name ( and the feature release 'One of our spies is missing')
Am I the only one in the dark here?