The first time I saw one of these was at a silent auction, and it took a while for me to figure out what it was.
It is a visual aid for a rodmaker, to help get the butt section line guides angularly aligned with the reel seat. It is lightweight and so can remain on the reel seat while the guides are wrapped.
I did not start this part with the intention of making a fixture. It was going the be a reel foot, but I did a poor job in locating the four holes for mounting to the frame. Now salvaged, it has at least some value.