See, even Nissan can't tell the difference between a 350z and a 370z. That 200cc displacement bump and weight savings are all smoke and mirrors; pay no attention to that man behind the curtain.
On a more serious note, if the last letter of your VIN is an M, then it's a 370Z. Nissan manufactured its 350Zs in Oppama/Tochigi (T), but manufactures all model years of the GT-R and 370Z in a factory dedicated to performance vehicles in
Tochigi (M).