Keeps small marbles from getting stuck when falling from Lower Zig Zag 3F to Lower Zig Zag 2F. I realize now I didn’t actually test it by seeing where marbles get stuck, but hopefully someone will test it before gluing.
2020 Jun 09: I discovered a circular piece should do the job
perfectly. I put the piece aside along with date and label into a bag for the Construction Crew to deliver later.