OK, well, I am glad you are happy with yours, let's start there.
Then let's also say - you provided some interesting info...a horn from the early 2000's of a brand nobody, today, has ever heard of...and according to what you dug up...it was made in Taiwan.
That's interesting, really it is because by 2000 many Taiwanese factories had gotten their game down on sax fabrication. So it makes sax nerds like myself wonder "I wonder where t came from exactly ? (factory)"
But this stuff:
" I have a black alto with gold keys. The metal on mine is silver and twice as heavy as a brass sax. The silver metal is called nickel/silver. They added nickel to the brass on some of the horns. It gives them a much darker/ mellow tone. "
Regarding the finish imparting a sonic character - I would invite you to stick around the Forum for a while....and maybe do some searches for " finish effect on tone" or some such semantics like that. Then read those threads.
Because, although for whatever reason people still choose to debate this (probably because manufacturers STILL choose to CLAIM this)...the general consensus, when looked upon in a controlled, logical manner...is that sax FINISH does not effect tonality.
So, IF your sax is darker-toned than others you have played, it is because the design specifications of the body, bow, and/or neck are of dimensions, design, etc. which give it a darker tone. Not because it is nickel-silver finish.
Next, you said that it's a black sax you have there. Then you said it's nickelsilver alloy.
Nickelsilver is silvery/nickel colored, not black. So...your finish might be black nickel plate, like a Keilwerth. That would be a plated finish, however, not an alloy mic but plating electroplated ON TO bare brass.
You say that something is added to the brass alloy which makes it nickelsilver alloy. This is so, yup- varying the %ages of copper, zinc, and nickel can create a nickelsilver alloy. Brass on the other hand is usually copper + zinc.
But if your sax is black...and has always been black, that is not because it is nickelsilver alloy (which is not black). It is POSSIBLE it is some sorta black plating plated ONTO a nickelsilver body...but, boy, that would be pretty unusual I think ? Someone can correct me....(?)
So...are you CERTAIN it is pure nickel silver all the way thru ? The only way to confirm this is to actually go at a key or spot on the body or neck very aggressively with a buffing wheel and see if the silvery finish maintains or if after some abrasion....you start seeing brass beneath (which would indicate plating over brass).
But this is still complicated by you saying the color is black...and you probably would NOT wanna do THAT test ...because if it ends up the black would buff off (it probably would, eventually) then you just scarred your finish.
Thirdly, the weight of a horn has a BUNCH of variables, some being : The metal gauge of the body and neck, the weight of the keywork and post mounts, the gauge of the metal of the keys.
So, while it is possible that the finish or alloy can contribute to its weight as well...
the only way you would be able to say that "because of the (nickel silver ?) finish/alloy this sax is Heavier than others"...would be to weigh one of the exact same model in a different finish or alloy (silverplated, bare brass, brass lacquered) and compare the weights.
The fact that one model of one brand of sax weighs more than another model of another brand of sax...could very well be because of, again, different design, different specifications, different metal gauges. Nothing to do with whether the sax metal is of a different alloy or a different finish.
The YAS 23 is a pretty light horn, for example, and of course is brass lacquer with nickelplated keys.
Yet the Conn 24M,for example... is a hecka heavy mo'fo...I mean, one picks one up and immediately says "wha !?"...and it also has brass lacquer finish and nickelplated keys.
So the weight has nothing to do with the metal finish, and probabaly little or nothing to do with or alloy, because I'm a' gonna bet the brass alloy of Conn in the 80's and Yama in the 80's was not dramatically different in composition. It probably has a lot to do with the body metal gauge, the design of the key mechanisms, the metal gauge of the keys, and the construction design of the furniture on the tube which affixes the keys to the body.
Mind posting a photo of yours ? Just out of curiosity...