Hi Andy,
This may help:
I can't find anything in the 650 workshop manual*. But the Ghost parts book gives the same part number for the chain as for the 750s (i.e LV 04100 30000 007).
In which case it is:
5/8" 135 ORS - 110 M.
P.s. Yep, this seems to be on the right track. Have just found this reference in the 668 owner's manual: Regina 135 ORS-A. No mention of the number of links, tho'.
* I don't think there ever was workshop manual for the 668s.
HTH
Craig