These Monarch / Davey valve adapters go in 40mm Ecopure sand filters, some Reliance (black Valve) & Davey filters.
There is a fine thread on the left Image. It has 6 threads & a course thread style on the right which has 4 threads. This is on the larger diameter thread only.
The smaller diameter threads are the same.
NOTE: The finer thread adapter is NO LONGER AVAILABLE - We do have an option...
If you have the fine adapter you will need to replace the complete union here and get it plumbed in