PVC reducer - insert it into a 50mm or 40mm elbow/T-joint/coupling, and the other side has a 1 1/4" thread.
Specification
Dimensions
Thread
1 1/4"
Inner...
Gluing/bonding connection with 50mm pipe.
Pool check valve to prevent water leakage from places above the pool level (filtration, pump motor.. etc.).
The valve has a spring....