There seems to be a little misinformation being exchanged on this thread.
First off, the 505/506 option did not exist prior to 1986 (model year 1987). ALL factory slantnose cars prior to that were Sonderwunsch (Special Wishes optioned) cars and carried no option labels stating so on the vehicle description label located on the underside of the front bonnet. Simply put, the VIN and engine SN will tell you absolutely nothing in terms of its authenticity.
It takes a great deal of investigative work on the car in question to determine if it is truly a Sonderwunsch Flachbau (1981-1986 MY).
1987-1989 Model year cars are much easier to ascertain as the genuine article. The bonnet label will state 505 or 506 and Porsche has records of VIN's built with the option. But even here, there are some 505/506 cars that went through the Exclusive Programme that had some special "tweeking" by Porsche technicians with features that were not on the options list.
For the most accurate information we have been able to come up with so far, we've put together a thread that hopefully, will give you some basis to work from.
Ultimate Slantnose, 505, 506 Flachbau thread