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Ann, You ask whether you can use a Nikon SB-22 with a Leica M6 TTL. I am using a Nikon SB-24 with a Leica M6. I use a wide piece of auto striping tape to cover all but the center contact on the hot shoe of the M6. I have checked the triggering voltage of the SB-24 and it is 9 volts, which I have found safe. Other older flashes may have trigger voltages of 250 volts+ (Vivitar 283). I set the SB-24 on auto. Of course, any and all mfrs, in this case Nikon and Leica would not own up to any responsibility if something goes wrong.
Hi.. I've been using the M6TTL for about 2 years now... I guess you're looking for TTL options on your flash/camera...? Then you are stuck with the SF20 or you could buy a Metz adapter to fit most of the Metz..
p.s. The advantage of the SF-20 (or the Metz with adaptor) is the full TTL automation. This allows for very, very easy fill-in flash. Just dial in -1 or -2 on the flash unit. This lets you use the Leica in the places where it excels -- low, ambient light with just a touch of fill to brighten the main subject.