There were no 993 (Turbo Look) Wheels in a 8.5" There was however a 996 Turbo (Turbo Twist) Wheel from the 996 GT2 in 8.5". The 996 GT2 had 8.5" and 12" x 18 Wheels.

Part Number should be 996-362-136-31 ET40 if it is actually a Factory 8.5x18 Turbo Twist.

Unless it is a Replica, then it could be any size that the many different manufacturers produce. In which case, it will not have any Porsche Part numbers on it, possible say Made in Italy, and weigh as much a 928.

 

What he means by not correct is that 265/40/18 is quite a bit larger overall diameter than the stock size.
You speedo will be off by about 5-6%, you're gearing will be longer, and your car will feel slower, because the tire has a bigger diameter.

http://www.1010tires.com/TireSizeCalculator.asp

 

with colorado roads so ****y i dont really care if it make car slower. the pot holes suck and its quicker then most already.