Is that also used on the ground strap as well?
There are 3 ground points for 65-66 engine bay harnesses where tooth washers are used... 1 at the voltage regulator, one at the firewall ground and one at the rad support in the corner by the battery.
As stated, yes. All three are 34940-S36 for 65 and 66. The difference for 1964 Mustangs is a -S8 finish. Apparently Ford changed the finish to facilitate conductivity.
The original washers had a wide tooth pattern, which are somewhat difficult to find.
Quite possible, I have a few of those on the shelf. Typically they referred to as "9 tooth" as used for 64; 65 and up are 12 tooth. The "technician" in me said use a new one and the ones I took off my 65 and 66 cars were the of the 12 tooth variety.
Also, note that the tooth washer goes on the screw last before installation, I see this installed incorrectly on a lot of show cars...
The theory is multiple contact points on grounds, as in between the ring lug and the body. The external toth lock washer is not used to retain the screw.
.. although I probably see more show cars completely missing the washers.
Without a guide or drawing, it is simply forgotten. The Mustang Assembly Manuals do have a use.
Jim