Summary

This is a level to align the dartboard using the single 20 or single 3 field.
Should fit firmly in all modern boards with "blade wires".

The cutout is fitted to the following levels from ebay:
https://www.ebay.de/itm/304209221704
https://www.ebay.de/itm/284398892895
"4 pcs Magnetic Bubble Spirit Level Small Portable Mini Tool Leveling M2J5"