A copper fitting is a small, cheap part that carries a lot of load. It holds pressure, carries drinking water, and sits inside a wall where nobody sees it until it weeps. Most homeowners never look at one closely. A competent homeowner with a multimeter and a flashlight can change that, and the habit pays off the next time a plumber, a supply counter, or a relative hands you a bag of parts.
ACHR News has a firsthand feature on NIBCO's copper plant in Virginia, and it is worth reading if you want to know where the parts on your shelf come from. The summary says the piece looks at the "products HVAC and plumbing contractors rely on." Those products come out of a process, and that process leaves marks you can read.
What the stamp tells you
Start with the marking on the fitting body or the box it shipped in. Look for the manufacturer name, the nominal size, and a lead-free indicator on any fitting meant for potable water. Many fittings also carry a pressure or temperature rating. You do not need to memorize a standards library. You need to confirm that the part in your hand matches the part on the invoice, and that the invoice matches the job.
Copper tube has its own labeling. The tube is printed along its length with the size and a letter designation, usually K, L, or M. Type K has the thickest wall, type L is the common choice for interior runs, and type M is thinner. Do not assume a replacement section is the same type as the original because the fitting seats on it. If the wall thickness differs, the joint can go together cleanly and still be the weakest point in the run.
Measure before you cut. A tube gauge or a set of calipers will confirm the outside diameter. A fitting rated for one size can seat on a slightly different tube and look correct while it leaks.
Press and push-to-connect parts
Press and push-to-connect fittings speed up repairs, and they show up often on residential work. Their markings usually list the pressure rating and the tube size they accept. Cut the tube square, deburr the end, and mark the insertion depth with a pen before you push. After the joint seats, give it a firm pull by hand to confirm it holds. Do not pressure-test a joint you have not checked for depth, because a shallow seat is the usual reason a good-looking joint fails later.
Soldered joints and the spool
If you solder, the lead-free requirement applies to both the solder and the fittings used on drinking water lines, so read the spool label as well as the fitting. Clean the tube and the fitting interior until the copper is bright, apply flux, and heat the fitting rather than the solder. Let the joint cool without moving it. Keep the spool wrapper with your project notes, since it is the record of what went into the joint.
Keep the paper with the pipe
Put the invoice, the box, or a photo of the stamped part in a folder near the main shutoff or the water heater. Write down the brand, the fitting type, the tube size, and the location of each joint on a simple sketch of the run. When a supplier substitutes a part, the packing slip is your evidence. If a joint fails years from now, that sketch tells the next technician where to look without opening more drywall than necessary.
What the plant story does and does not tell you
The NIBCO feature is about a manufacturing site, not a repair procedure, and it says nothing about your specific joint. What it does show is that the parts in your walls come from people, processes, and investments that leave evidence behind. Your job runs the same chain in reverse. Read the evidence on the part, match it to the tube and the job, and write it down. Check the stamp, check the tube, check the seat, and keep the paper with the pipe.