Because Sir Roger Moore deemed that no good photos exist of him as a child, the black-and-white boyhood picture of Brett Sinclair in the opening credits is that of his son Geoffrey Moore.
Sir Roger Moore quit smoking cigarettes in 1971, following a stern lecture from Tony Curtis on the set, which is a bit odd, as Curtis continued to smoke into the early 1980s. Update: Not that odd: Curtis was anti-tobacco but pro pot. Moore switched to cigars from cigarettes.
In one episode, Tony Curtis answers a hotel telephone with the words, "Bernard Schwartz? Never heard of him." "Bernard Schwartz" is Tony Curtis' real name.
Sinclair's car is a yellow 1969 Aston-Martin DBS (genuine reg. PPP 6H but usually seen with the plates BS 1 which was specially negotiated with UK circus proprietor Billy Smart, who owned it in reality); Wilde drives a red 1969 Dino 246 GT Ferrari (reg. MO22 1400). No, they are 1970 models as proven by the auction notes referenced elsewhere here.