John Wood guesses that the trend to use only odd (number of teeth) cogs might have been "...a factor of the smallest cog in use - when 13's became available racers quickly swapped out their 14-22's for 13-21's. Again, just a guess."
FWIW, the Campagnolo Gran Sport was marked "14 - 26 Denti" and many of us learned the hard way that it would skip with a 13. That derailleur was perhaps the last with the pulley cage pivot symmetrically between the two pulleys. The "brass" Record derailleur, which handily controls a 13, became available in the earliest 1960s.
And, of course, Real Racers don't ride Huret or Simplex, eh? :-)
Thus, the Wood postulate puts the date of the Odd Cog complex at early 1960s.
harvey sachs
mcLean va 22101