These dust caps are made to fit any standard 2nd stage hose and reg fitting. One use is to cap the oxygen solenoid and hose of your rebreather.
They come as a set 1 male and 1 female (one for the hose and one for the reg)
The dust cap for the REG is fitted with a seal which makes these dust caps water proof so the regs can be removed and washed or stored with out the worry of getting water, dust or other things that might damage the valve inside.