This is an attempt to reduce noisy back panel resonances for certain print head speeds.
The design is based on Stockbridge tuned mass dampers https://en.wikipedia.org/wiki/Stockbridge_damper
Purely empirical with zero quantitative theory behind it. It reduced noise for my printer some, so if you have the same problem, it might be worth a try.
The center hole is sized to be a somewhat tight fit for a M4 bolt, the side holes are supposed to be tapped with a M6 thread. May need to clamp while tapping to prevent layer separation.
Add some random M6 bolts for added weight.
The author marked this model as their own original creation.