Hi IU89- It appears based on the VIN that your vehicle is a 1984 Model Year. Here are two pieces of data from Porsche. From the factory's online listing, 655 Platinum Metallic is shown as an '84 paint code option (it's the 6th listing from the bottom of the page)...
Additionally, from the actual '84 US Color guide, it appears that Targa & Cab models with 655 also had the secondary code "U1V9" (The V9 apparently designating a black top)
Please note this additional string from a 1983 Cab with Platinum Metallic (see the "U1V9" designation on the owner's manual image)...
http://forums.pelicanparts.com/porsche-911-technical-forum/775688-question-paint-interior-codes-83-sc.html
As such, could the secondary "L999" designation actually have been for a unique cabriolet top fabric color other than black?