one general misunderstanding here.
xenon mainboards can be downgraded! full stop. NO matter what dashboard version you have. it does NOT matter if you have NXE or not!
the so called CB version is decisive, not the dashboard version or NXE or not.
if you have CB <1920 then you can downgrade !
even some early zephyrs are still downgradable (the ones that had no HDMI yet, but the new GPU coolers allready) basically all consoles manufactured until July/August 2007 without HDMI were downgradable and still are. that are millions of consoles.... even if 50% are dead, which i personally dont believe, there are still millions left.
here you can see the versions well:
Xbox360 revisionsand here are some sales numbers from 2007:
xbox360 sales until end of 2007thats a good enough number for a homebrew scene....