Is there a chance the low end screw was loose enough to vibrate out of position? I've seen this on SH engines and the fix was to wrap a few turns of Teflon tape on the threads to hold it. Be sure to use a very narrow strip no wider than the threads.

I don't remember if there is an O-ring on the OS engine low mixture screw or not. If there is it could be dry or cracked.

Glad you got it running anyway.