12B = built second week (B) of December (12) 1968
M40 is the option code for the TH400 trans.
Typically the transmission code wasn't put on the trim tags for US built cars, but it was for Canadian built cars. If your car was built in Canada, you can get a report on it with a listing of the option codes from Vintage Vehicle Services for a fee:
http://www.vintagevehicleservices.com/options.html
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