If your rebuild kit is from Jon H then the needle and seat is NOS. You are in good hands.
The difference you see in the used parts are that past rebuilders used the wrong parts.

The spring is like a shock absorber for rough terrain (farm truck bouncing around in a rough field) to prevent flooding.
Jeep has the same design.
The tube extension in the seat is for fuel sediments to settle rather than lodge in the needle and seat.

Is the idle mixture spring new or old?

YF 684S? Maybe you have a different model?

Last edited by buoymaker; 03/17/2022 12:51 AM.
"Adding CFM to a truck will only help at engine speeds you don't want to use."
"I found there was nothing to gain beyond 400 CFM."