It appears that the inductor is used specifically to shorten the long leg of the OCF so that the feedpoint is still in the right place electrically even though the physical size of one leg is different.
Generally OCFs work with roughly a 2:1 ratio for the two legs, but looking at the photos, it appears that they've loaded it such that both legs are now equal. Try two lengths of wire 18.5' long, then split one of them roughly in half and install the inductor. So long as that coil isn't a trap (is there a capacitor in that piece of pipe?), it's unlikely that placement of the inductor is critical.
Your antenna will look like:
----18.5' wire----[balun]---9.5' wire---[coil]---9.5' wire---
Check your SWR. If you're close but not quite, try moving the inductor a little closer or further away from the feed point and see if it dials in.