I found it in the 70-73, PET Main Group 9 Electrical equipment, Illustration 902-10, position (item) 23. This individual wire is also shown in the illustration for engine wiring harness pieces in the same section in the 74-77 PET but without a part number. 1974 may have been a transition year; my '76 has this brown coil ground wire molded into a female connector that plugs over both coil terminals and the wire is incorporated into the engine wiring harness.
Here is an image from a 1973.5 911T of what I believe is the 901.612.099.00 wire:
