I'm in the process of installing rear seat belts in my Oct 65 San Jose build 66 "K" Fastback (the rear interior is spread out all over the place). When I removed the old ones (they were tucked up under the seat) I found a different type of washer than what is called out in the 1966 Mustang Body Assembly Manual on page 41 (N6-8718). The manual calls out 381559-S2 7/16 special washer, a type with serrations on the flats, to be installed between the belt attachment plate (and plastic sleeve on the belt) and the body. What I found was a 7/16 internal toothed lock washer, 34921-S2, but installed between the head of the bolt and the belt attachment plate. In my accumulation of hardware, I've found a few of that type of internal tooth lockwasher still hanging onto the attaching bolt. I thought is was not a factory installed belt application but apparently it is as my other 66, a hardtop built two days before the "K" had the same thing. The only other place this size (7/16) internal tooth lock washer is used is on the special ground stud on 289CID engines with an automatic transmission as a wiring ground for the battery to the engine.
Jim
