Chris, thanks for the reply.

I've had a number of Polaroid MP-4 shutters (so marked), have one in front of me, and have the catalog. There's not a diaphragm or diaphragm control lever in any of the shutters and the only place on them that the word Copal appears is on the shutter speed strip on the side of the shutter. All self-cocking shutters, top speed 1/125. The catalog says nothing about a diaphragm but the illustration shows a shutter with no aperture control lever.

You have have a different Polaroid Copal shutter that's had an MP-4 shutter's face plate put on it. Or, perhaps, a non-Polaroid Copal Press #1 with, for some odd reason, and MP-4 face plate.