Four Flies on Grey Velvet (1971)
The killer's identity/clue to his identity can be seen in the victim's eyes.
poster on the wall
This film is mentioned in this this documentry.
mentioned in overview on Argento's giallo's
Four Flies on Grey Velvet is playing at a theatre in the opening scene
Luigi Cozzi's memories about the production of "Four Flies on grey Velvet"
Theatrical trailer shown