I have bought 6 nuts from three different sources: 1 from Amana via Amazon, a pack of 4 from QWORK via Amazon, and 1 from MariTool bought direct. All of these have the retaining ring that the collet snaps into milled off center to the degree that they are unsusable.
So does anyone have a reliable source for ER20 Collet nuts?
Make sure the nuts are balanced. They will have a drill mark or two where some material was removed on the heavy side. A spindle running at up to 24k rpm needs balanced nuts.
The Maritool will be balanced for sure. But not certain about the others.
EDIT: I took a look at the two Amazon links you provided - neither of those would be acceptable to me. They clearly are not balanced and likely cheap Chinese offerings. Stick with Maritool - not that expensive, made in the USA, and balanced.