If you're looking at this you most likely know exactly what you're looking at.
These are from BlindAssasin111's GB back in 2018.
These were designed to accompany the Nexus MX Sliders, while they fix the majority of the binding issues (especially with stabilised keys) you may still need to widen the opening to ensure no keys bind.
These are great for fixing damaged Alps switches and mounting MX-mounted artisan keycaps.
Here is a link to the original GB post with more info.
Note: B-stock items may carry some scratches and signs of previous use. Please see the product picture for examples of this.