Andrew, how would she forget that she held the camera at waist level?
Are you assuming that this was a moment in time for which every detail would be permanently seared in Marina's memory? I take pictures with my camera, with cameras belonging to others. I don't necessarily remember what camera I used let alone how many photos I took. She remembered taking a photo of her husband with his guns and that there was no one else in the backyard taking photos. When shown that there was more than one photo that appeared to be at a very similar time and in the same location with the same conditions, she agreed that she must have taken more than one. She still couldn't remember taking more than one but she agreed she did only because the photo was shown to her. 133A is very similar to 133C. What makes you think she would not have said the same thing if shown 133C?
The Imperial reflex is operated like a Hasselblad, not like a 35mm eye level camera, and unlike the Hasselblad does not have an eye view finder. She said she held it to up her eyes. She should also have remembered that the image in the camera was upside down, which it is in a reflex camera.
Was the image reversed left to right or upside down?
Wikipedia says that TLR cameras with the waist-level finder reversed left and right. That makes sense, because you are looking at the upside-down reflection from the mirror and the mirror is reflecting the focused image from the viewing lens which is inverted (i.e upside down and reversed left-right)
Both very hard to forget.
But easy to not remember in the first place. Our brain does not store details that are unimportant so that it has room to store the important things. At the time, the detail of where she held the camera was of no importance to her. Why would she remember? She admitted she took at least one of the photos. They were all taken with the Imperial Reflex 620 camera so it is apparent that she was wrong on thinking it was taken with a camera that had an eye-level viewfinder.