Some of this discussion caught my attention as the USAF required a similar paint dab, yellow per their specifications, indicating that the torque was inspected on bearings on the azimuth and elevation sections of the 60 foot dish antennas we designed and built under contract (Ford Aerospace and Communications).
In the "1968 Mustang Chassis Assembly Manual" (AM0025) on pages 66 thru 68, are several uses of -S100 finish on hardware used on steering gear with torque specifications also spelled out. Unfortunately, the actual meaning of the "-S100" is not stated, but I believe that in this case, that it indicates the red dye requirement. There are notes referencing the colors used to identify ends of tie rod assemblies, directions of bolt installation for engine options, and adjustment requirements on these pages. If you have a 1968 Mustang, you should get a copy of this manual. Pictures do not specify torque.
The are a lot of smudged hardware part numbers on these manual's pages. In my research for the Mustang Hardware Spreadsheet (See this forums library), I have determined most of the correct part numbers. Send a PM if interested.
(Note: The drawings also state "Mustang and Cougar".)
Jim