A small circle next to the 7 means the seventh is diminished.
If the letter d is capital, it means the triad is major (D-F#-A) and the 7th on top is a diminished one (C flat) but if the d is lowercase, it means that you have a diminished 7th chord (d-f-a flat-c flat).
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If the letter d is capital, it means the triad is major (D-F#-A) and the 7th on top is a diminished one (C flat) but if the d is lowercase, it means that you have a diminished 7th chord (d-f-a flat-c flat).
At least, that's how I remember it.......