Regarding the modern, made in Japan, Zeiss ZM branded lenses for Leica M mount...Some have serial numbers starting with 51 but most appear to have serial numbers starting with 15 - even for the same model lens (eg, 50mm f/2 Planar).I thought I read some explanation for this but I cannot find it any more (and my memory is spotty). I tried asking the mfgr, Cosina but have not received an answer. Does any body know what the different prefixes signify?TiA.