Date of introduction of TO-92 transistors
Edit

Thanks for the picture, by any chance are their any identifying text left on the packages or the boards? Just a shot in the dark, but I would say that board was from the aprox: 1972 to 1985. But it depends on so many other factors. And it also my not be original to the organ it'self. 


Original Answer


  I'm trying to date a circuit from an organ.


It would be the easiest to just look up the organ... 


  Date of introduction of TO-92 transistors. Approximately when did
  these transistors first appear?


The TO-92 spec was introduced in the 1960's (I believe but can't find a source,) and became a standard by JEDEC Solid State Technology Association years later.

FYI, none of this information will help you date the organ's board!

It depends on the actual model of the transistor and a few other factors. Semiconductor material (date first used): 


the metalloids germanium (1947) and silicon (1954)
in amorphous, polycrystalline and monocrystalline form
the compounds gallium arsenide (1966)
silicon carbide (1997)
the alloy silicon-germanium (1989)
the allotrope of carbon graphene (research ongoing since 2004)