That particular ring, by itself, is the aperture stop. The normal operating mode of the Land Camera (150, at least) was a coordinated exposure system that coupled particular shutter speeds and apertures using the EV system.
From the Land Camera 150 user manual,
The shutter dial adjusts the camera to the right combination of shutter speed and lens opening for the particular scene. Your Polaroid Exposure Meter PR-23A tells you which number to use, and you just turn the shutter dial until that number show isn the little window above the lens. Your camera has numbers running from 10 to 17. Notice the letters "EV" above the window. This means that your camera uses "EV" (Exposure Value) numbers,
Later in the manual, the Conversion Table For Lens Openings shows the f-number and shutter speed for the dialed in EV setting:
#10 = f/8.8 at 1/12 sec.
#11 = f/8.8 at 1/25 sec.
#12 = f/8.8 at 1/50 sec.
#13 = f/8.8 at 1/100 sec.
#14 = f/12.5 at 1/100 sec.
#15 = f/17.5 at 1/100 sec.
#16 = f/25 at 1/100 sec.
#17 = f/35 at 1/100 sec.
Notice the long arc on the bottom half of the wheel in your image, that doesn't obscure the lens. That corresponds to the fully open aperture of ƒ/8.8 in the #10-#13 EV settings in the table.
The gearing around the rim of the wheel is driven by the shutter dial just visible to the upper left on the other side of the exposure mechanism in the photo.