b_t_p,
The two gear cables run continuously from the handlebar grip changer right into the ex box (cable box) where the inners wrap round and terminate in the cable pulley. The cable pulley has a 8mm socket in the centre to grip the 8mm hexagonal peg which protrudes from the external transfer box. Setting everything to gear 14 and fastening the ex box onto the external transfer box hides all the moving parts from dirt etc. and sets it up for gear changing via the handlebar grip changer.
The only spare part I carry on camping trips is a full length gear cable inner. I have heard of one ex box being bent, presumably by a collision with a rock or tree, but I don't carry one as a spare.
I hope this helps.
julk