From October, 1958 a vehicle id plate was placed behind passenger front seat. This plate can tell you the exact date of manufacture of your vehicle, factory options (M-Options), export destination, paint and upholstery colors, and model number and layout. It is usualy called M-Plate.
M-Plate info:
05 7: Date of manufacture
430 A42 080 108: Optional extras called M-Options
UT: Export destination 2312: Model number/Paint Finish 523907: Paint/Upholstery 7000881: Year/VIN#

As you can see, our bus left the VW Hannover Factory in July, the 5th, 1966. It was for the Tourist Program of the US export buses and all the peculiaritys (US spec bumpers, sealed beam highlights, red rear indicators...) are there. The A42 designation means it was conceived as a Westfalia camper ordered from a VW dealer, with no exterior fitments. With louvered windows instead the normal ones and with the very very cool option of the front split seats, aka walkthrough. It was painted in Pearlwhite.

M-Options details:

M430 North America export package, including:
  M020 MPH speedometer
  M024 Sealed beam head lamps and red tail lights
  M025 US-spec bumpers and 6 pop-out windows
  M090 Safety glass windshield
  M175 Overriders
  M620 12v system (standard from August, 1966)
  M621 12v warning light relay
A42 SO-42 Westfalia interior with no exterior fitments (Westfalia code)
M080 Split front seats (walk-through)
M108 Camping Equipment, without side windows (5 louvered windows instead of all solid/popout windows)
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info taken from: thesamba