IIRC NHSP used 45s. S&W 4566, 4566TSW and Sig 220s. I don't recall them ever using 9mms, although they may these days with the Famous But Incompetent having declared 9mm to be deadly again.
That 5906 you have there is a very early production example. Very possible it was a T&E example. A letter might provide more info. Or maybe call the 1800 number and give them the serial number. See what they are willing to tell. Good luck! Regards 18DAI