Steve, my back boxes have holes in them where the id plate is tack welded on. Could get a s/s set made up but it is the cats that will define the system and MGR ones (from Wilcox) will soon be NLA. Cats don't last forever so I've decided to replace the whole exhaust system with NOS MGR bits. If they last another 15 years then the system will probably survive my last breath.
Someone in MGR at some point in the past told me they were 419 and if they are not then they must be top class mild steel (might be galvanised mild steel?). 15 years for MS is very, very, very good.
Kev