Buying a K5

Is there a serial number that should avoided? Could somebody please clarify when the good K5's started to emerge.Do we have a datum point?
Just in case -you never know
Regards
Chris

Im really not sure why people go by the serial number .. the manufacture date is much more precise.

Anything from 2011 should be fine although a few have crept through the system after this date.

Thanks Mike.
Didn't realise it was by year of manufacture.
That's useful to know.
Regards
Chris