NCC-1017. I know people have wondered why a Constitution class Starship would have such a low registry number instead of something in the 1700's.
I had a thought...maybe the Constellation used to be a Saladin class ship, you know, with a single warp nacelle in place of the secondary hull.
Then, at some point, the Constellation was upgraded, the secondary hull and twin warp nacelles were added to convert it into a Constitution class starship. It kept the same name and registry that it had before.
May be reaching a little but I think it would be a good explanation of why the registry number is so low compared to other Constitution class ships.