Some early armstrongs were marked 'model number' 'letter' 'four digit serial'
In your case model 104, 's', 2510, should date to ~1970.
Compare the marking on this model 90 to the one on the 104 you came across.
http://www.conn-selmer.com/en-us/about/ ... armstrong/
$1500 is way too much for an armstrong 104. $150 in reasonably good shape could make sense, but with an old instrument appearances can be deceiving. An instrument that is pretty shiny might be about ready to fall apart, one that is dull as a bone could mechanically be in great shape, and be a pleasure to play.
If you're shopping on goodwill, you'll have no idea what you're buying. Might cost more to fix up than it would be worth.
https://www.shopgoodwill.com/auctions/W ... 51271.html
Unless you really know what you're doing a repair shop or music store selling a used flute (plenty of good options online if you don't have any local shops you like) is probably going to be the cheaper option than buying on goodwill, craigslist, or personal sellers on ebay - some music stores sell quality instruments on the platform. Remember just because someone says it's in good shape doesn't mean it is. Doesn't have to be deception, lots of people know little to nothing about their instruments.
https://www.fluteland.com/board/viewtop ... f=4&t=5859