Many Macintosh II, IIx and IIfx models shipped with hard drive trays designed for 5-¼" drives, even if they shipped with a standard 3-½" drive. This is a bracket that lets you mount a BlueSCSI version 2 directly to the tray, without having to find (or make) an intermediary adapter.
Since the screw holes in the BlueSCSI v2 changed locations during a revision, this bracket accommodates both variants. The standoffs on the corners are sized to fit normal hard drive screws, while you can use M2.5 or M3 screws to attach the BlueSCSI. It prints in about an hour on my Prusa Mini.
The author marked this model as their own original creation.