Idle Adjustment Screw?

[bonedaddy]

For some unfathomable reason my idle adjustment screw ('75 911S) disappeared. I determined that it's a 6 mm. 1.0 pitch and temporarily replaced it with a screw from the hardware store. I was wondering if there was some means that the factory used to keep it from backing out or moving from vibration. I tried to determine what the original looks like, but can't seem to find it on the handy dandy exploded views on the Pelican site, or in my other manuals. Any help? Thanks.