Dan, my understanding is that the K type (K3) had a two piece iron cast bracket and without aluminium clamps or steel support piece .The ones you show appear to be standard and modern repros.
I do have the K3 style castings in stock yet to machine up when I can come top for air!
You are correct about the cast iron body mounting brackets, they are unique to K types, also the body mounting brackets which fit over the front of the rear springs are unique to K types.
Thanks for the response. Id love to see a picture if anyone has one. Rich, please keep me informed of your progress unless you want to sell just the castings. Best Dan