Ed,

As I mentioned above, the most likely chord is a Bb chord (since A# is equivalent to Bb). If that's the case, it would make the chord that Leo mentioned a triad based on the flattened 7th of the C major scale. This is a relatively popular chord these days.

Here's a small article about it

https://www.songwriting.net/blog/topic/flat-seven-chord

Regards,
Noel


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