Hi Geoff,
Your helmet certainly sounds fine - as requested I've attached a couple of pics of mine. Interestingly mine was never issued, so has never been whitened. Internally its never been worn. Apologies for the rather poor quality of the photos, they were quickly taken in the kitchen!
I attach a pic of the rear two vent holes, I wouldn't put any great importance if your doesn't have these, they were possibly a manufacturing variation, perhaps on helmets supplied to the force shortly before the amalgamation, which could explain why this one never saw service.