It's not fitted on all models. Early D2 has all the internals but no linkage but then it was done away with on later models. The transfer box internals could be bought for later models that were produced without the internals.
The linkage and internals are available as has already been said and can be got from Ashcrofts.
I put CDL L/H shifter off 93V8 disco on my 2001V8 disco 2. Works great. Had to bend L/H linkage over 1 1/4" to match up. CDL linkage matched up with a minimum of adjustment.
Hi i had parts fitted from Ashcrofts, and did my own disco 1 linkage conversion, a lot of tweaking, but got there in the end,Traction control and diff lock mind blowing!! and thats in deep mud with a sankey trailer fully loaded hanging off the back,the only thing that could beat that is a tractor