Symmetric models of ZF and GSet for large G

Hi, this has been bugging me lately, and my copy of Blass and Scedrov's AMS Memoir on order for interlibrary loan is being slow. ... Recall that the topos of G-sets for a large group G gives us a model of ZF via its internal logic (use a universe if desired). Has anyone written down the precise relation to symmetric models as defined by set theorists? Roughly speaking, these are submodels of generic extensions that are fixed pointwise by a family of subgroups of the automorphism group of the generic filter (if I have the basic idea correct). Perhaps this is in Blass-Scedrov, and I just need to be patient... Regards, David Roberts [For admin and other information see: http://www.mta.ca/~cat-dist/ ]