Winchester has had a pretty standard, dated 9mm headstamp format for LE rounds for years. An example would be “WCC +P+ 91” with the “WCC +P+” across the top and the date at the bottom, originally with the numbers of the date close together, and latter, spread apart somewhat. This headstamp style never had a NATO mark. I would think, although I don’t know, that they are just continuing that style of headstamp for the FBI.
I don’t know about the .40. There is a Federal “military-type” headstamp for .40 with just the initials “F C” at the top and a two digit date, such as “09” at the bottom, but I don’t know about Winchester. The Federal loads may have been made for the U.S. Coast Guard, or other Federal Agencies that use the .40.
A lot of you guys are associated with AFTE (I am not, although I have provided them with information in the past, on a few occasions). I would think that someone at AFTE could find out these headstamps with nothing more than a phone call to the Bureau at Quantico, where the range is, or to the FBI Crime Lab. I would do it, but I don’t have the credentials to expect an answer on what might be construed as a “sensitive” LE subject.