I'm not certain the top photo is actually from the film - but I think it was taken during filming (probably a Mick Rock photo?) The band definitely looks satin here to me - but if it's not from the film it could have changed (as other things did).
The second photo is a screen grab and does lend itself to the sequin band theory a bit more. This photo also shows the underside of her tail coat lapel is gold (not black).
And the last shot is a screen grab just to show the size and shape from a straight-on view. Don't make it too small - that thing goes ear to ear!
Edited to add - I think the trading card photo (taken on set) also shows the band to look like a scrunched satin neck band.