NH baler broke needles..why, how to fix?

Last year I bought a NH 273 baler that had been in the shed for about 10 years. I went thru it thoroughly, greased it, slowly started it up. adjusted the knotters and after about 15 broken bales it made 30 or 40 good ones. Good knots, and nice straight bales.

Suddenly it broke both needles, smashed them into 3 pieces, and spun one of the bill hooks (probably when the tip of the needle hit it).

First, what did I do wrong? what should I have checked? Is the needle stop the key to not breaking needles? If the needle stop is in place to protect the needles,does the needle stop go into the chamber every time the needles cycle?

Next I recently bought a set of used needles. Do I just bolt them on or do they need to be adjusted?

Then I have to fix the knotter, could the hit on the bill hook have forced the billhook gear into the wrong position on the drive gear/wheel? It does not seem to have sheared the billhook pin.

Finally, how should I restart it to be sure the problem is fixed, especially since it was baling well then just suddenly broke the needles?

One more question, if you bought a used baler at auction and could not talk to the previous owner, how would you go about starting to bale with it?

Thanks for all your help and input.

This my current idea of what happened, first the knotter brake did not hold tight enough to pull the needles back out of the chamber, and then the plunger stop was frozen and did not go into the chamber, so the plunger broke the needles due to 2 malfunctions? What do you think?