I'm guessing your Motors Manual is a 1950 version. It would be interesting to find the first printing of a Motors manual that includes any mention of torque values.
It appears the first patent for a torque wrench was 2,159,354, granted in 1939. It's interesting that the patent application mentions "the tightening of cylinder head nuts on internal combustion engines or assembling precision devices such as air craft engines and the like"
I'm really curious when torque standards were finally adopted by the auto industry.