It's not what it seems. According to Kensington, there is no master key for these locks. They work correctly, but the master key is not possible.
π Ultimate Security Bundle: Maclocks MBPR15BUN Case & Cable Lock for MacBook Pro Retina 15-Inch Laptops
9 Review
Kensington Microsaver Universal Computer Key Lock and Cable Security System 64162a for Desktops
9 Review
Improve your iMac Security with the Kensington SafeDome Secure Lock (K64962US)
11 Review
Secure Your Laptop with the π Kensington K64638WW ClickSafe Keyed Twin Laptop Lock
12 Review