IMO you "stole" it
regardless of the condition.
I'm always willing to learn new things, but I wasn't aware that Amperex ever made 6550's. Maybe the name like so many of the old classic tube makers has been prostituted and stamped on Chinese and/or Russian tubes?
I think all of the Sunns from that era had black tolex, but the black grill is
extremely rare;
so rare in fact that if yours left the factory with black grill cloth it's probably the only one
ever to do so.
Use the search function and you'll get some info about your new amp. It's essentially a SS rectified 1000S/1200S, so info you get about those models should be helpful to you.
Also you will find threads about where to get the proper grill cloth, as well as speaker combinations other members use with their similar amps.
Great score! I wish I could have snapped it up, those deals don't happen very often anymore. Of course if it were a
2000S, I'd have probably booked a flight instead of posting the listing here!