How do you tell the difference between a PSI D and a PSI D 400?

I'm replacing the injector hat for my PSI D - the old one didn't like being on the track at 200mph ;-)
I've been asked whether I have a standard Model D or a Model D 400. I'm assuming that mine is a standard Model D, but just to be sure, how do you tell the difference? i.e. what should we measure?

Martyn Jones