OK, let me know if this is the correct or intended procedure. It seems to produce a plausible-looking output file, but I'm not sure if it's actually right.
3dVol2Surf apparently expects as part of its input that you will edit the spec file to include only the surfaces you care about, and that two of those surface specs be marked with MappingRef = SAME.
If this is correct, it's not at all obvious from the help or the SUMA usage doc. If this is an input condition it would be *really* helpful if this was in the 3dVol2Surf doc.
Graham