When I bought my triton a few years ago it had sat for 10 years or so, had the same black 'band'
I tried scrubbing it, scraping, end about everything else I could find. What worked was castrol superclean, (not made by castrol anymore but still made) and it surprised me with how well it worked, just rinse it well.
Since then I've used it on nearly everything I need to clean, from boats to motorcycles, it will discolor aluminum and brass/bronze if you don't rinse it well. Also, it can't be beat for cleaning
varnish and 'crud' from old carbs.
Ken.