Not quite true.Most 600 amp circuit breakers that I have seen have twin 500 kcmls lugs. If you know the circuit breaker brand can you go to the breaker people to see what they come with. The generator guy I deal with knows what the machines come with. Maybe the non supply chain issues are creating the uncertainty. Whatever lugs the breaker company has on the shelf is what you will get.
Square D standards:
Two holes, 400 MCM max, Cu only.
A common private labeled breaker:
Page 99 states lug holes (I think 2) are for 3/0 to 500 MCM aluminum, or 3/0 to 350 MCM copper. So 500 MCM copper probably fits but it’s not rated for it.
Breaker, Lug Kit, Front Connection, 2 x 4/0 AWG - 350 MCM CU, 2 x 300 - 500 MCM AL, Cat #: TCAL63, Mfr: ABB
4/0-350 copper, 300-500 aluminum.
So I’m curious, which brand of breaker has two 500 MCM lugs at 600 A? None of the common ones do.