Mark plays a tape recording of himself aged 5, made by his father who had every room in the house wired. Domestic tape recorders were not available until the late 1940s, when Mark would would have been at least 10.
During the opening sequence we can see a shot of Mark's camera, showing that it is somewhat concealed in his coat, at chest level or lower. In the subsequent "first person" shots of the prostitute (supposedly showing what's being seen through the camera's viewfinder) it seems to have moved to eye level.
When the trunk with the body is opened, the woman scans the body from left to right (from our perspective), and as soon as she sees the right side, she stares and screams. However, when we finally see the body inside, the head is on the left side.
When Mark is viewing his own 16 mm version of the police carrying the body of the prostitute from her flat, there are three bystanders shown between the camera and the police and gurney. When the bystanders move, they cast shadows on to the scene, revealing that they are actually standing in front of a motion backdrop. Thus Mark's film is of another film.