How did you permanently mark USCG NO.

Well, even fiberglass boats have metal in places. It doesn't have to be engraved on the hull.

true but they are glassed in place during the built. A metal placard with screws is not permanent as they’re easily removed.

Being a biker I call it pride in your ride, but same thing.

Actually it doesn't have to be permanently affixed. It must be installed so that removal is obvious.

No, that's generally stamped on the transom by the manufacturer. This is a documentation number (federal registration).

• The vessel must have the official number permanently affixed in block-type Arabic numerals of not less than 3 inches in height, preceded by the letters “NO .” on some clearly visible interior integral structural part of the vessel.
  
• https://www.dco.uscg.mil/Our-Organi...ter/National-Vessel-Documentation-Center-FAQ/

That's fine as far as it goes but you left off the last sentence that recognizes that nothing is permanent. A couple of screw holes from placement of a plaque would comply:
"The official number assigned to documented vessels, preceded by the abbreviation "NO." must be marked in block-type Arabic numerals at least three inches high on some clearly visible interior structural part of the hull. The number must be permanently affixed so that alteration, removal, or replacement would be obvious and cause some scarring or damage to the surrounding hull area."
Again, if you're a certified / commercial vessel you'll be held to a more stringent standard and who's going to debate semantics with a CG inspector who can hold up their certification. But I've seen them done on recreational vessels with stick on letters, permanent marker and plaques attached with 3M tape and I've never heard of any recreational vessel charged.

https://www.dco.uscg.mil/Our-Organi...nal-Vessel-Documentation-Center-FAQ/#anchor12

The reason I'm bringing this up is I had an issue with this very same situation, where the boat had a nice plate scribed into starboard and mounted to the stringer with 12 screws. They made me grind the screw heads, with a drill to where a screwdriver couldn't remove them. Which was a major pain in the ass. The stick on decals with a nice coat of resin looks clean and meets all parameters and is easy and only way to do it on a fiberglass vessel that complies, unless you want to whittle the numbers INTO the fiberglass, which I wouldn't recommend.

All grinding off the heads would mean is that you'd use a drill or prybar instead of a screw driver to remove them. Sounds like someone didn't like you. But you're correct that there's a right way and a wrong way, and then there's the technically legal way which is why you never hear of a case unless it's tacked onto other charges or someone really ticks someone off.

Ours is routed out numbers on a piece of mahogany, that is glued and screwed to one of the below deck bulkheads. It's a 1973 boat, so I'm guessing it has passed more than a few Coast Guard examinations without any problems.

Thanks for sharing Ralph, that sounds like a winner. If one did try to remove it there would be holes and adhesive. Of course the rule is not exactly specific as to the fastening of a sign or the routing of numuber or varnishing over numbers. And one would think a 1973 boat on the water full time has been inspected but, I bet So. Florida is the inspection capital of US waters not Mississippi.

Well, it's also in a place in the boat you have to crawl to( between the forward fuel tank and a bulkhead) , so I'm waiting to see if any Coastie actually crawls in to look when I tell him where it is! I have to crawl in there to change out the water system filter and I hate when I have to do that, LOL, because you have to contort so much to get to it!