The Leica M6 TTL, produced 1998-2002, succeeded the M6 Classic and added through-the-lens flash metering, a 2mm taller top plate to house the electronics, and a larger shutter dial that turns in the direction of the meter arrows.
It keeps the mechanical cloth shutter 1s-1/1000s (battery-independent), adds TTL flash with compatible units, and offers 0.58x, 0.72x and 0.85x finder magnifications with framelines from 28 to 135mm. Weight is about 585g.
Photographers divide between the Classic and the TTL on handling: the TTL's reversed, larger shutter dial and flash capability against the Classic's slimmer top plate and purist proportions. Functionally the TTL is the more capable camera.
Check the flash TTL circuitry and meter both work, as electronic faults are the main value risk. The 0.85x version is sought by 50mm-and-longer shooters and prices accordingly; special editions (Millennium, LHSA) trade well above standard bodies.