Rare doesn't necessarily mean valuable. That being said, I personally disdain the "billboard" markings on current guns.
Do you know of any specific examples of the value of guns that have left the factory with imperfections such as missing "roll marks" or missing "anchor" stamp or wrong "caliber" stamped on the barrel. Colt is awfully interested in getting this King Cobra back!!!!History tells us that gun errors (unlike with stamps and coins) add little or nothing to the value. This is more of a curiosity than anything else. Most error guns I have seen carry 5%-10% premium at best.
Just like when my agency received a shipment of new S&W revolvers in the late 1980s. When another instructor and I tested them before their being issued we found one with a smoothbore barrel...no rifling at all. I called S&W about it once we got back from the range and was told it had to have passed through the hands of at least twelve or more inspectors. He said "We want that gun back...I want to show it to a lot of people."I bet they want it back to use as an example in an additional training class for their millinneal employees.
I own 1 Z gun as well which was an employee gun. The Z stamp was used when Colt sold a gun as used. The guns were sold with no warranty. The Z was to let Colt know if the gun ever came back with a warranty concern that the gun was sold with no warranty. Z stamp guns could have been demos, show guns, factory salesman samples, custom employee guns and even customer buy backs. All got run through product service before they were sold to make sure they were ok and that (product service) is who stamped the Z on the guns.If it were me, I would contact Colt for a Replacement. I would not have even left the shop with it.
I have a Ned Buntline commemorative 45 NF nickel from about 1979. It was unfired when I got it 10 years ago. I took the stocks off to find the grip frame is numbered to another gun. It is what it is, poor quality control.
I have 2 “Z” guns as well, these were colts sold with known flaws which I am guessing were cosmetic and shoot fine. Both guns were bought below the going rate and are fine with me.