Looking for cleats to match my vintage Mavic pedals
I don't think those are the 646, but the cleat you probably still want to use is the Look Delta cleat. Go to a bike shop and try the delta cleat in there. 

In the olden days, everyone was rebadging Look cleats as their own. 
The 646 (and many old pedals) use Look Delta cleats (aka the original/traditional/ARC Look cleat; NOT the Keo cleat). The 646 should use the black cleat since it has float adjustment on it. 

Picture of the Delta cleat (From REI):