Apart from the foiled Signature Series and Signature Edition text, if you compare the 2006 one to a regular one, the Knights logo is white in bits that is black in a regular one, and the 2007 ones have white down the bottom where the signature goes, I think on the regular ones they are blue or black there, but certainly another colour.
No certificate, but you'd have to have a printing press to do a counterfeit, you couldn't just alter a regular tazo.
The Sharks played really well, well done. Knights look awful this season ... either the drug bust has had a real effect on the players, or Stone is struggling as a coach at NRL level (why oh why did we ever let Brian Smith go?!?!), but the side plays like they met for the first time 5 minutes before kick-off!
Tazo autos are rare to come across from my experience with them. They can go to up to $200 (abit rediculous for a tazo IMO). Not sure who would have them up for sale on this forum, maybe check ebay on a regular basis for them.
Cheers
Brplat