Securon 500/30 is your best bet for a series 2. They are manufactured brand new and come with all the fittings, which will fit the P6 anchor points. The buckle design isn't "period correct" but they're more likely to save you in an accident.
If you have an original inertia reel buckle, it is possible to transpose the 'tongue' part onto the new belt system, which is what I've done.