Propane OPD valves have threads on the inside and the outside. Other caps do not protect the inside of the valve. This is the most important part of the valve, in that contamination will prevent the cylinder from properly mating with the utilization equipment, resulting in a leak. Leaks can lead to fires resulting in personal injury and property damage.
Other valve plugs currently sold will allow insects to get inside and build nests. Sand, grit, snow, ice and other debris will also accumulate in the mouth of the valve.
This will not happen with this cap!
The inside cap is not leak tight.
Any miniscule leakage will travel around the threads between the inner cap and exit at the vent holes placed in the cap.
The valve plug LPG & NH3 Supply sells will completely protect the inside of the valve from any contamination issues.