It is 68mm. I know what you mean about conflicting information...
This is the part number 36136783536. (It says it is 64,5mm in some places)
It is the new style with chrome edge. Also the old emblem is flat, the new is convex.
It fits almost every modern BMW rim
Fits my 220 19" great anywho, looks great!
I bought mine here: http://www.salesafter.eu/