I'd be willing to bet you're right . most , if not all , departments put an inventory(?) number on their weapons . Is the # on other spots ?

Or it might be someone slapped another slide on the 5904 at some time . I myself have a 639 I swap the slide out with a 3906 slide I bought . Mostly because I got the slide cheap and also it has adjustable sights . It's h*ll getting old .