Red A/C box assembly mark

[IronCanine]

I have spent a bit of time trying to suss out a pattern and reason for the red mark that appears on some A/C boxes just above the Harrison decal. They're doesn't seem to be a pattern with regard to assembly plants. And they also appear episodically on GM cars of this era. Member F.A.A.benny reports seeing them frequently in Pontiacs. Does it indicate the system has been charged? Is it simply an assembly check? I figured it was time to throw it to the collective and see if anyone-maybe some of you that dabble in other worlds like vettes, Chevelles, and the like-may have some insight. I have seen them on St. Louis trucks, so I'm putting the mark on our build...but I've seen more trucks that don't have the mark. Anybody got the scoop?...or at least some ideas?

Thanks for any insight.

Brad