There's no such thing as a shanked snaffle.
Any bit with shanks is a curb. A snaffle is any unshanked bit with a 1:1 ratio of pressure (as in, you pull with one pound of pressure, the horse feels one pound of pressure).
A curb is any bit where the ratio is greater than 1:1. The shanks multiply any pressure on them by however long the shank is. So, on a bit with a five inch shank, when you pull with a pound of pressure, the horse feels 5 pounds of pressure, pull with 2 pounds, horse feels 10 pounds of pressure, etc.
It's a very common misconception, don't worry!
It's mostly due to the fact that TT's are sold as "Tom Thumb 'Snaffles'" because of the mouthpiece (which a lot of people call a "snaffle mouth"). They really should be called "Tom Thumb Curbs" though!
Hopefully that all makes sense. :)
Anyway, sorry for taking over your thread with bit talk!