Definitely flat for everything - I prefer a TeeJet XR or Turbo TeeJet for foliar apps like PGR, and air induction (AI) for pre-emergent and soil conditioners. There is more discussion in this thread.
The nozzles are threaded - they drop into a Quick TeeJet cap. The TeeJet nozzle body is threaded. Or you can usually just drop a TeeJet nozzle into a Chapin cap.
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