I have a question for all you guys that may be able to solve a problem for me!

I recently acquired a second hand, mint condition, lovely piece of kit called a smithshop Joint-Matic.
I see some of you guys on web forums rave about this piece of kit which is great.
The problem is that it came in bits and now putting it together I have what looks like a major piece left over.....
Can anyone tell me what it is called, what does it do and where does it fit.
Feel free to jump in and shed any light on my situation.
Please see images before.
Many thanks in advance for your time. -Good Luck!

Regards Chris Webber from the UK