Turbo Hydra-matic tranny speedometer not working

[cppursell]

Hi everyone, I'm hoping you can help my dilemma.

Ever since I've had my '67 Chevy C10, the speedometer hasn't worked.

I'm pretty sure I have a 350 Turbo Hydra-matic transmission.
I looked under the truck and I noticed that the speedo cable is attached to this box-looking device which connects to the transmission. I thought that was just a third-party gear reducer or something, but it seems to be part of the transmission, 'cause if I take that away, there's no way to directly connect the speedo cable to the tranny. Looking at the box-thing, there seems to be a missing part inside the end that plugs into the tranny, and that's probably why the speedometer doesn't work.
Are you familiar with this box device, is it part of the transmission, how can I fix it?

I hope I was clear enough...
I don't know what I need to get the speedometer working, but if you can give me any pointers, that'd be great.
Thanks!