If your serial number starts with AYY, AYP or any prefix close to those it is likely a 19-5 from a Diplomatic Security Service order.
The DSS ordered several runs of RB 2.5 inch and RB 4 inch 19-5's.
The ONI guns were 19-3 and IIRC the NIS 19's were a mix of 19-3 and 19-4's.........been awhile and I don't remember.
I do remember that the majority of the ONI guns were later sent to the FBI for their use. Those RB 19's were demilled and destroyed - with a few converted to "red guns" for training - during the Clinton regime. To my knowledge no ONI guns have survived to make it into civilian hands.
There are actually a few NIS RB 4 inch 19's still in service today. And more than a few DSS 19's, although they are in foreign Embassy service. Not CONUS.
Nice 19 you have.