I think he is referring to a patch box.By saying "box in the stock for gears" are you saying it has something like a place for a crank to go that would drive gears?
There were several Sharps carbines known today as "Coffee-Mill Sharps" which had a coffee grinder built into the butt stock.
These are very rare and costly with a "good" one selling for $17,500 in 2007.
Like all rare guns there are a number of counterfeit ones on the market so be wary of any you might come across.
I have never heard of any company making modern repros that have ever offered one of these models for sale.
Then, on the other hand, maybe your talking about something else?