FXRoyal 400

Shot sting velocities will give you a clue. If you have a bell curve, then probably no reg. If you have a pretty straight shot string, probably so. 
If you want 100% definitive Yes/No, you'll have to do some dismantling. The risk with dismantling is that you risk damaging a few o-rings. 
It will be behind part "G" if you have one. There is a sequence to taking it apart properly. 
Tom