The Vongola is a really big organization. Lal already said that aside from the original XXX number of animal box weapons by the three inventors, they were able to make storage boxes, weapons etc.
So I'm guessing there has to be someone in the Vongola that has learned about the boxes and is able to make their own. The future Hibari went around learning about the boxes - those information by be sent to the Vongola allowing them to replicate an animal box.
To be honest, you're thinking too much into it. :C
I mean, it's the Vongola we're talking about. Surely they have the technology and skills to make box weapons too.
And Nutz is a very bad translation for his name. I really did laugh when I read him calling him Nutz. Peanuts would've been better. (not to mention cuter!)