REdding Sizing Die ID

[Pastprime]

I received a Redding sizing die today but no instructions and I'm not sure which one it is. It's for a .223 and has a couple bushings. The side of the die body has ".223 Rem B/NK U1" stamped on it. Looks like the decapping rod has no expander ball. Can someone please tell me which model this is??

[Steelhead]

I believe it's an S series neck die.

B/FL is full length

[Pastprime]

I believe it's an S series neck die.

B/FL is full length

Thank you. I wondered if NK meant neck.