I was under the hood of my 88 244 Dl today doing an oil change. My dad came out and asked me what this pluge was for on the passengers side of the firewall almost directly under the wiper arm on that side. I was curious, so I pulled it out and found threads for a screw. I,however think that I might have done something very stuiped, in that I could have caused a water leak. I put it back, but I'm not shure I got it in there as good as as it was, as the head has just the smallest amount of gap between it's self and the firewall. What is this hole/thread for?

Thanks for your help, and happy bricking.