From the Anvil Spring Unit Cassette
Overall Size: 2" x 1 3/8" (51mm x 35mm)
It converts unsprung handles to spring action. It is cut into the door below the back plate either on one or two sides. So if you have a poor lock and you want our handles to work perfectly this is the answer.
This cassette is designed to increase the tension and performance of an old or weak lock.