Don’t know. I’m assuming the first gen one (16# iirc)
I’ve got another nucleus gen 1 that is running the factory spring and have zero issues with. You just need to remove the grease from the bolt and spring and make sure you call timney and get their seat that’s setup for the newer custom actions
the timney 510’s work fine due to the shorter seat vs the Calvin Elite 2 stage in blue.
Timney sent the other sear at no cost when I called them