Does NF(U+?) have $\beta$-models? A $\beta$-model of a set theory (or higher-order-arithmetic theory) is a model $M$ of that theory which is correct about well-foundedness: if $x\in M$ is an illfounded relation, then there is some $a\in M$ which is a subset of the domain of $x$ with no minimal element.
I know a bit about $\beta$-models of ZFC- and $Z_2$-like theories - in particular, $\beta$-models of ZFC are just well-founded models of ZFC - but I've realized that embarrassingly I know nothing whatsoever about $\beta$-models of NF-like theories. The existence of a "reasonable" set theory without $\beta$-models would be amazing, so I'm sure that $(i)$ there are easy ways to construct $\beta$-models of NFU (or even strengthenings like NFU + Infinity + Choice) and $(ii)$ there are no major reasons to be more skeptical of the existence of a $\beta$-model of NF than of the mere consistency of NF.
That said, I still don't see how to whip them up. So:

How does one construct a $\beta$-model of NFU?

I'm especially interested in $\beta$-models of strong extensions of NFU (like NFU + Choice + Infinity + "Cantorian sets"). I'm also interested in heuristic arguments about why (I assume!) the $\beta$-consistency of NF should be equiplausible with the consistency of NF.
 A: (Thank you, Alice, for getting me to look at this) Before i answer i think i need a bit of clarification on what exactly a $\beta$-model is.  On the face of it NFU cannot have a model which speaks about wellfoundedness without forked tongue, beco's the ordinals of any model of NF(U) are illfounded.  That is to say, it is a theorem of NF(U) that there is (an explicitly definable) proper class of ordinals with no least member.  (It may be that Rosser-Wang ``Nonstandard models for formal logics'' addresses your interests .. JSL some time early 1950's - they discuss NF in some detail)   But it may be that you mean something subtly different.
  best wishes

     tf

A: There are no $\beta$ models of NFU.  The natural order relation on the ordinals of NFU in a given model M is ill-founded, externally, but M thinks it is well-founded.  So there are collections of ordinals of M which have no minimal element, but none of these are sets of the model M.  This is a classic result:  there is a paper about it, to which I should supply a reference when I am not on vacation away from my office.
