You will need the lighter of the two U springs to hold the standard trigger against the upper sear piece and stop it rattling forwards and backwards.

Correct, the heavy U spring is used to power the set trigger and is not required for the standard trigger.

You will see the upper 'leg' of the standard trigger fits between the two flanges hanging down from the sear piece. The fit between these parts is often quite slack, and therefore the spring is required to take up this slack and prevent a huge amount of free travel before the sear starts moving.

The lower leg of the spring should fit into the notch cut into the standard trigger.

I found that fitting the standard trigger resulted in a big increase in creep... to the point I felt it was a backward step. It really needs a set screw to take up the slack and reduce sear engagement.

I persevered with tweaking the set trigger on both my ZKK's and ended up with an acceptably crisp light release when set. When used unset, it was only marginally better than the standard trigger.

If anyone develops a replacement trigger for the ZKK, it will fix the Achille's Heel of what is otherwise a great action!

Not sure if the CZ550 magnum trigger assembly can be retro-fitted as the action is very similar to the ZKK602?

Can you post an image of the split washer... don't recall seeing one on any of my ZKK's... someone may have slipped one under the rear action screw to limit overtravel of the screw end into the boltway above. Common cause of bolt-binding trouble on the ZKK's...

Cheers

TM