My first thought is that it might be a lubrication issue; my second thought is that the effect is the opposite of what I’d expect, given the way the lens aperture pin operates - noting that when the pin is pressed in, it stops the lens down and, when no pressure is on the pin, spring tension of the aperture mechanism opens the aperture and the pin moves outwards.
Mechanically, the whole process is: when the shutter release is pressed, the mirror is first driven upwards under spring tension and the plate moves forward to push the aperture pin inwards. The mirror reaching the top of its travel causes the first curtain to be released. At some time interval, based on the shutter speed, the second curtain is released and, as it travels, a shaft also revolves which has a “pin” affixed to it. When the second curtain completes its travel, the pin on this shaft pushes aside a lever which is holding the mirror up. The mirror then begins to drop to its normal down position. At sometime during this latter process of the mirror being released, the aperture plate should be released as well.
With lack of lubrication and no lens mounted, I could understand how this plate might fail to return to its rearward position and also hang the mirror.
What confuses me is that, if there is some lack of lubrication with this plate, the presence of a lens with the aperture pin pushing back on the plate should actually make it more likely for the plate to move back and more likely for the mirror to come down.
