In my quest for a Cougar (need to sell my Mustang first…) I see a lot of ads claiming “numbers matching”. I assume they mean numbers matching engine and trans. The cars I’m looking at have mostly been '67s and a few 68’s. I wasn’t aware that Ford VIN stamped engines and trans in '67, I thought it didn’t start until '68. Unless it was a Mercury thing? I’m very new to Cougars, so be nice!
Matching numbers often means different things to different people. Like the terms restored, rebuilt, factory, stock and so on. So its always good to ask someone what they mean when they use the term so that you figure out what they are including or excluding in their definition
For some it can include everything, every casting date and steel metal panel date to others who consider it just that the engine size matches the VIN number 
All engines and trans were stamped in 68 (a new federal requirement) where in 67 only some engines and four seed transmissions were to be stamped the transmission number related to a national auto thief regulations/request For had been doing this (for four speeds) since 62-63
Hope this help s
My understanding is that it was hit and miss prior to 1968, when it became a Federal Law to VIN stamp major drivetrain components (i.e. engine & trans). So on a '67, I would think that when they state “numbers matching” they mean that the date codes are all appropriate for the assembly date of the vehicle.
When in doubt, you can always ask the seller for a picture of the VIN stamps 
*Yeah, what Jeff said!
Here are a couple videos on the subject.
https://www.youtube.com/watch?v=BxWh6R6OyUA&t=5s