Good question. I never really remember what they’re called, but we have spoken about them many times over the years.
They are specialty companies and individuals, who are in someway legally connected to the DMVs around the country, so can do all the legwork necessary without having to involve yourself with the DMV directly.
Cost money, but in the past, the costs have been very reasonable for the results.
There are a couple of well-documented horror stories here on the forums. One of the members went through a years long struggle to get his vehicle back after trying to register it. And after putting in bucket loads of money into a restoration.
It was legally his in the end. But the road to get there was long and arduous.