Sorry, I’ll expand on this some more.
The M guns bought on the FBI contract have the nDLC small parts. USMS, DSS, and I believe DEA are getting guns off the FBI contract. There may be more agencies, not sure.
My agency issues the 19M, but they were not part of the FBI contract. They’re basically just early Gen5’s, and don’t have the nDLC parts. @
BehindBlueI's agency did the same with 17M’s.
Lipsey’s sold 19M/17M’s that were not FBI spec. GT Distributors, and maybe other blue label distributors, briefly sold blue label 19M’s that were FBI spec and had the nDLC parts.
Additionally, Glock has redesigned the FBI 19M, making both internal and external changes. I’ve not seen any “in the wild,” but have seen a photo of one.