I believe mine, on Parker built #131,which is welded to a substantial U shaped mount of the same material,as you say bolted through the 'tunnel', is of stainless steel.
I have had it cleaned back to metal from time to time and it certainly has a matt SS appearance rather than a bronze type colour.It has no signs of surface corrosion and has a pair of anodes on the same cross bolt.I inherited these.I apply Trilux every year.I haven't tried it with a magnet.I have read of the limitations of stainless used below seawater level - at the moment it is happily doing it's duty and apparently in good structural order..
Peter