Some history on the American Thread Company.
http://www.textilehistory.org/AmericanThread.html
When I was growing up in CT, the Willimantic plant was still in operation. I still have lots of their books, yarns, and tons of their thread, but none of it says quilting thread. I use what I have for piecing with no hesitation at all.
C&C (who separately were part of American Thread) has recently reintroduced Star thread for quilting, but at 50 wt, it sounds much thinner than the vintage spool you have. The new stuff is nice, a mercerized long staple Egyptian cotton thread. I like it a lot and it's not as linty as some Coats threads. I digress.
How far into the spool did you go? Does the thread perhaps soften up after the first two or three 'aged' layers?