This is the R3 replacement water chamber for Respironics M series humidifiers. If your current humidifier requires a black latch, this is the humidifier chamber lid for you.
This water chamber is not compatible with Respironics PR System One Heated Humidifiers.
Check your humidifier. On the dial, under the number five, there will be a thin line that indicates that you need an R3. If there's no line, you have the R2.