I have never seen a guide like that, and I don’t understand from the picture how it would work. I believe it was not supplied by the C. S. Bell Co., especially since this is a school bell and not a church bell. Most church bell guides were two in line pulleys between which the rope passed. They were mounted in an iron mounting which was placed in a rectangular hole in the floor of the belfry just beneath the edge of the wheel so the rope passed directly down through the guide.
Looking at your full picture of the frame your bell is sitting on it kind of looks like there may have been a second one of those mystery loops on the other side. There appears to be a nut in the exact same spot on the right side of the pic. Additionally, I see there is a hole on top of the loop. To me it looks like that may have been a place for a bar to be secured for some reason.