Stainless steel quick disconnects(QD) are designed for higher pressure than us car guys will be using but will hold up much better to corrosion than zinc or brass.  Also, it looks much better and really doesn't cost much more.

You really don't need this part.  The MTM Foam Cannon comes with a zinc plated/brass plug, but I prefer stainless.

• Inlet = 1/4" Male Plug for (Quick Disconnect) QD:  This 1/4" male plug inserts into a female QD coupler.
• Outlet = 1/4" (National Pipe Thread) NPT Male:  This 1/4" male threaded end screws into a female 1/4" connection.

Connection Flow:

Mosmatic 20" Lance Outlet -> 1/4" Male QD Coupler -> 1/4" Male Plug -> MTM Foam Cannon Inlet 

*Think... Water Coming In to Water Going Out

Warranty Information:
MTM offers a 60 day warranty for any possible defects with their products. Info about their warranty can be found Here.