questions on huck finn, please, help me!

hello everybody,

i hope you can help. i am doing a presentation on the book and i got two questions in mind:

1. i remember reading that the end of the book was written and added later. is that correct or am i mistaking? if it's right, can anyone give me a relieable source for that?

2. what about the year of publication in the us? 1885. because someone manipulated a picture in the book? does anyone have a source for that?

would be great!!!! :)
thanks a lot!!!

*preciousone