Some time in the mid to late 70's Porsche changed to the black Polyamid lines shown above. This or a similar product is now being used worldwide by most auto manufacturers.
This is a very durable material that is semi-rigid but it takes a special barbed fitting that is one-way and considered permanent. The only way to re-use this type of fitting is to melt the Poly tube off the fitting. If you try to cut it you run the risk of damaging the barbs.
Some folks say the fittings can be installed on new replacement tube with common hand tools, but that has not been my experience. I had to have special tools made at a machine shop in order to do the assembly correctly.
Up to present, I have heard of no failures of any kind with the black Polyamid line, only failures at the hose connections which are permanent in the factory configuration.
I have recently been advised that this Polyamid line is even suitable for E85 fuels now being preferred by the turbo models.
That is why I started making my new ones with threaded fittings as in photo above.
Len