Reason for my asking since I haven't owned a CR or GR prefix *** mod yet. You have some to compare!
I am wondering if they used the original inner band on the mkIII-type conversions, then modified the forend for use below the outer band, or if they changed over to the slightly larger diameter inner band & screw to take advantage of the newer MkIII location.
Or...was the original inner band swaged open as to not bind on the barrel if moved back? That would seem like more work than neccesary but I tossed it out there to cover all the options.
I see your sporter has what appears to be the original inner band and screw judging by its location. I'm thinking that should you use one of your spares you may want to change the inner band & screw to take advantage of the existing inletting.