[njk53]

There is a high probability these are altered production heads that got out of GM.

The 1X before a production part number was and still is a designator so the part can be tracked in the GM system. Then the part could be put on a bill of materials list to be used on a development/early build vehicle or placed on an existing test vehicle. If the change to the part proves out the release engineer requests a new part number for production.

The 1X generally means there was a modification to the production part that was big enough to require a drawing change. The change did not have to be major just enough to alter the drawing. The AC means there were 3 changes made to the prototype part.

Usually the 1X part is taken off of the vehicle and given back to the release engineer who usually would scrap them. I have seen a lot of good stuff get thrown out but it looks like this set of heads got away.

The production part number behind the 1X is really what you have with an alteration.