Somewhere in the process in most states someone from the state or with authority from the state has to physically check the numbers and that the tag they are on is mounted with the correct fasteners. The check harder for stolen vehicles thing is behind that. I've heard of a few guys who got rejected by the dmv because the vehicle had the wrong fasteners holding the plate.
Still having a clear image of the plate as it sits on the cab in you build book if you are building one from pieces like Joedoe and a few others might do with their frame swap trucks where that cab off the grain truck works great on your build helps with the process as sometimes does a photo of the cab or truck where it was sittiing when you bought it plus a printed out copy of the add from the net if it was advertised on FB, Craigslist or another site. Plus if you didn't get a title a legit looking bill of sale. You can get bill of sale forms off the net and print them out to take along on the quest.
I tossed in the view from the backyard this moorning when I want out to get a clear shot of the door post tag on the 51 1-1/2 ton tag.
Here is the how I found it shot. When I went up and bought the two trucks.
Back when you could buy both of them for 100 for the pair. And then spend twice that getting them off the hill. That was just a few months before scrap metal when sky high That is more if you are doing the assembled from parts with no title though.