The Polaroid lens is either MP3 or MP4. I have an MP-3 with that lens. It is self cocking as you go to trigger it. There is a second port where a cable release with a long plunger can go in, that allows the lens aperure blades to withdraw and shutter to open to allow more light in and help you focus on the material you are working on copying.
The slow speds do get gummy with time in my experience with this lens.