Red Assembly Mark on A/C boxes

[IronCanine]

On some A/C boxes there was a red paint mark from the factory. At least I interpret it as paint. I have seen many more boxes without a mark than ones with a mark. I believe it to be a Harrison mark, not a final assembly plant mark. But I am speculating. It appears on other GM makes of the era and on vehicles from different assembly plants. The location of the mark can vary. Most all I have seen were at the top front, but I have seen it at the bottom and side. Here are a few pics I've collected from the board at large and from members directly. First two pics are St. Louis trucks, fifth pic is Flint. The fourth pic is from our build and an attempt to recreate this mark. I have had conversations with members about the meaning of this mark and found no consensus. I didn't have much luck noodling around the Corvette, Chevelle or Camaro forums either. F.A.A.Benny would be a good voice to offer an informed perspective on this.

I'm not aware of other threads discussing this and thought a discussion and pics might be of use to someone down the road (and I have an affinity for factory processes/markings). If nothing else maybe we can get another thread with factory underhood A/C pics.