Peregrine gives Sparrow's bowling shirt to Steed, telling him that the blood stains are from Rex. In 1964, blood tests could only show that the blood was of the same blood group as Rex's. Only beginning in the 1980s could DNA tests match blood with a certain individual.
In the opening scenes Sally tells James that "It is 1964". Soon after an exterior view is shown with an XP Falcon sedan parked in the street (Wikipedia says XPs were released in March 1965).