I also have an N.P.D. marked 3953, all I have been able to determine at this point was that in 2005 or 2006 the Newark, NJ Police Department issued a "S&W 9mm" but then change to a different maker. Their S&W 9mm's were put into storage. In 2007 Jersey City PD had some issues with older S&W 9mm's and NPD loaned them some of their "S&W 9mm's" until the problematic guns could be repaired or replaced. The guns were returned and went back into storage. Newark PD has not responded to my request for info regarding these "S&W 9mm's" and if they were 3953's or not. I was told that at one point they issued both 6946's and 3953's, but I could not confirm that. But I do know that NPD has a tendency to mark their guns and store them for some time before finally letting them go.
The only department's I am aware of that ever issued or approved of the 3953 are Canadian agencies, a couple department's in New Jersey, a couple in Massachusetts, and the NYPD. Most were only issued to detective and other plain clothes positions. My quest for info continues!
ETA: The seller of my 3953 purchased from a wholesaler and did not know what N.P.D. stood for.