It is likely a half length curve of O, a standard offering. It would take 16 halves to make a circle. (normally 8) . Each has 22.5° of arc vs standard 45°.
You can make these by simply cutting a curve in half and adding a tie....and pins .. to the halfs that need them. (or soldering to another track to stop rail twist/roll at the joint)
Standard O min curve is 31". I don't think that ad line means 0-31.5" dia. The ¼" extra would barely be perceptible even with a measuring tape. I guess you could assemble such an exacting circle if you combed tons of track for the very longest variances and grouped them. It may be of use for using 0-27 and 0-31.5 together parallel, witbout trains clipping each other in curves.