You had one on the coolant drain plug? If you are not the original owner of the car maybe the previous person took it out (if it was cracked) and never replaced it?

 

Apologies on reviving an old thread, but I wanted to add my .02 cents since I recently stripped the plastic screw head on the heater relief valve myself.
You can either order the entire replacement assembly (which comes with the plastic relief valve and body assembly that attaches between the 2 heater hoses at about $17 or you can purchase the radiator flush valve located on the underside of the radiator, called the **** - water drain part #21440-5V000 for around $7) The part in the previous picture where the bag is upside down is the same thing.

There is in fact a very small O-ring on the heater relief valve screw that you will need to gently remove using a small flat tip jeweler's screwdriver or similar and replace on the new plastic screw too. I'm not sure how much of a difference it makes to have this, but since it was on the old one, I just transferred it over to the new screw. I also put a small amount of grease on the threads to reduce coolant leaking out around the screw which was the reason that I was trying to overtighten in the first place. Make sure to use the correct size cross-tip screwdriver too and don't overtighten it!

Picture #1: new screw right out of the Nissan bag with part #
Picture #2: both screws (the old screw is on the left, notice the O-ring at the end)
Picture #3: old screw with O-ring removed

 

Barry did you replace your coolant? I need to do mine and would love to talk to you about any issues you ran across. Give me a call when you can.