Outer Limits 4x4 Board

Couldn't find anything on search - what year did the viscous Centre diff come out?

Thank in advance

My mate has it in his 1997 model 80 series, its a 40th anniversary.
I dont think it was in any models before 1996.
Or should I say any models besides the 40th anniversary.

You notice these dont have the centre difflock switch fitted on the dashboard. People do fit the switch later as the wires/plugs are there just taped up.