Updated: 11-Jul-2023
All of these prototypes were designed and built by Karmann Coachworks.
Locale: 
Golf I Prototypes
One of the first protoypes:
This prototype debuted December 7, 1976:
The above prototype, amazingly, has survived the decades. Initially housed at the Karmann factory museum in Osnabrück, it now resides at VW's AutoMuseum in Wolfsburg, Germany:
(1979 production car is in the background)
1977 Prototype
The 1976 design was rejected by Volkswagen, so Karmann went back to the proverbial drawing board and came up with Prototype MW-77, which in itself underwent a number of variations. One of these variants removed the C-pillar notch:
Eventually, MW-77 began to appear like the car we know and love today:
^Note the larger rear plastic window, chrome door handles, snap attachments for the convertible top boot, and lower placement of the Karmann badges.
1979 Debut
After undergoing some refinements, the final design of MW-77 was presented, approved, and began production in mid-year 1978. These pre-production models were used for in-house testing, promotional material, and press junkets. The final iteration of the Golf Cabriolet made its official public debut in March 1979 at the Geneva Auto Show:
Mass production of the Golf Cabriolet for consumers began in February, 1979.
Golf II Prototype
In 1985, Karmann presented a Golf II Cabriolet prototype. This prototype was ultimately rejected by both Volkswagen and Karmann for being too expensive to put into production. Sadly, the photos are all that remain. This prototype, lacking drivetrain components and real tail lights, was destroyed shortly after VW & Karmann made the final decision to nix the project.
