c80n:
Check the HKCR/.3ds/(Default) then. It should have the file "3dsfile" value, same as HKCR/.3dz/(Default) nearby.
HKCR/3dsfile/ShellEx/IconHander/(Default) should be {FC548317-B187-4404-B7EC-FA9EE87EFE84}, which is the GUID for my class, but since the icons for 3dz files display properly...