Ipcress and Quiller

Two films with themes by John Barry, both Cold War spy thrillers, released one year apart. Both use the cimbalom, a Hungarian folk instrument like a cross between a xylophone and a zither (the other unusual instrument heard in the Quiller theme is a musical saw).

The Ipcress File (Sidney J. Furie, 1965) features a brainwashing machine called The Mind Processor, inspired by Illinois Institute of Technology's Knowledge Box, later to develop into the vr CAVE. The electronic soundtrack for The Mind Processor was by Brian Hodgson at the BBC Radiophonic Workshop. 
IPCRESS: Induction of Psycho-Neuroses by Conditioned Reflex with Stress
Best British Film of 1965 (BAFTA)
Listen to The Ipcress File theme

Michael Caine as Harry Palmer

The Quiller Memorandum (Michael Anderson, 1966) is a potentially great film with a fatal flaw.

"Noted for its plot twists and the portrayal of Quiller as refreshingly vulnerable and occasionally inept"* and a "deliberately paced but engrossing script"* by Harold Pinter, starring roles for Alec Guinness and Max von Sydow, location filming in what was then West Berlin with a capital W - but: the miscasting of George Segal in the leading role. Aside from the fact that he's better known for (and suited to) light and humorous parts, there is the unalluded to anomaly of an American being part of the British secret service.
                                              *Lee Pfeiffer, Quiller article in Britannica

Max von Sydow as Oktober, George Segal as Quiller

 Listen to The Quiller Memorandum theme

Bonus track. Matt Monro, former London bus driver, and the man of whom Frank Sinatra said "If I had to choose three of the finest male vocalists in the singing business, Matt would be one of them" recorded a vocal version not used for the intro or end credits, but which is heard playing on the radio in Quiller's hotel.