My 12C 68 is just over 13". Original p/n is 3864327 superseded to 3923912 according to april 1970 PnA.
It took me a while to hunt it down in that dinosaur of a book [listed in the index as Retainer chassis wiring harness]
It's in grp 2.559, last entry and I quote:
65 - 69 All ....3923912 Retainer firewall wiring [13" O.L.] gen. and forward lamp harness to rad. tie bar [Note 1]
Note 1 reads - Loose trough type wire and hose retainer [rubber] use at cowl or radiator tie-bar locations as service replacement for similar retainers which are part of wiring harnesses.