This is most likely the minimum visibility (in metres), with the compass direction given as "//" (not possible to determine). A single slash is more common, i.e.
0650/. WMO Publication No. 306 - Manual On Codes - Volume I.1 - Part A, Section A, FM 15 METAR, FM 16 SPECI (emphasis added):
15.6 Groups VVVV VNVNVNVNDv
15.6.1 The group VVVV shall be used to report prevailing visibility.
15.6.2 Directional variation in visibility
When the horizontal visibility is not the same in different directions and when the minimum visibility is different from the prevailing visibility, and less than 1,500 metres or less than 50% of the prevailing visibility, and less than 5,000 metres, the group VNVNVNVNDv shall also be used to report the minimum visibility and, when possible, its general direction in relation to the aerodrome reference point indicated by reference to one of the eight points of the compass. If the minimum visibility is observed in more than one direction, the Dv shall represent the most operationally significant direction.