from the leica faq...
M6 / M6 TTL differences
To save battery juice, the larger shutter speed dial of the TTL features an "OFF" setting to remind you to turn the lightmeter off when stowing the camera. Unlike the older M6, merely setting the dial to B still leaves the meter on
The TTL s/speed dial rotation direction is opposite to that used on older Ms. A huge (and for some traumatic) change, this was done to let you turn the dial in the direction the lightmeter arrows point
The TTL light meter is -1EV more sensitive
The TTL lightmeter display has three red LEDs - under, correct, over. The older M6 only had a left arrow 'under' and right arrow 'over', which made it more difficult to judge how far you were from correct exposure
The TTL model is approx 2mm taller to make room for extra electronics. It's easy to spot the difference - look at the space above the tiny rangefinder window next to the "M6" engraving. For older Ms, the window is almost flush with the camera top. With the TTL (and M7!), there's a small top border
The TTL model dropped support for reloadable Leica film cassettes (although it's possible to use them if you replace the base-plate!)
For SCA-3000-compatible flash units, TTL flash control
The TTL model has the letters "TTL" and a lightning bolt engraved on the flash hotshoe (a dead give-away, no?)
People who actually use the camera with a flash(!) report it won't fire the flash if the s/speed is higher than X (1/50), or if camera battery is dead or missing