As far as I was aware PED applies to all pressure vessels that are made commercially, prototype or not. A pressure vessel is still a pressure vessel.

At 70BarG for volumes of:

Below 1 Litre - Sound Engineering Practice applies, No CE mark required
Above 1 Litre - Cat 1 applies, CE Mark is required
Above 3 Litre - Cat 2 applies, CE Mark is required and an approved notified body must be appointed.

When the pressure rises, the volume for Cat 2 falls so Cat 2 kicks in earlier (at 100Bar working pressure, Cat 2 volume drops to 2.5 Litre)