Photo credit: Maidstone Museum & Bentlif Art Gallery
I think that this painting is not by Allan Ramsay but by his fellow Scottish artist Katherine Read. Their styles were quite similar. I believe it is the painting on which John Finlayson based his mezzotint published 15 May 1771. This mezzotint by John Finlayson, after Katherine Read of Maria Gunning can be seen on the National Portrait Gallery website https://bit.ly/3qlKz5g.
The painting is in a similar pose with a few minor alterations, with for example the flower shifted from the hair to the neck and the lace around the neck has been omitted and the lace at the top of the dress has been simplified. I believe such alterations were common when doing prints of portraits. What is similar is the hairstyle, the pearl earring, the pose, the string of pearls. Perhaps the painting looks more ‘feminine’ also than the style of Allan Ramsay.
I doubt it. The original Katherine Read engraved by Finlayson is a pastel, which you can find at J.612.199 in the online Dictonary of pastellists http://www.pastellists.com
The scale of the portrait and the handling of the features and the dress, as well as the composition itself, do not fit comfortably with Katherine Read's work. And generally engravings of famous people followed the original painting fairly closely. I think we must look elsewhere for a fairly minor artist whom I fear will defy identification.
I agree with Jacob that the print is quite unlikely to have deviated so much from the original, and that this is by a minor hand. Needless to say, this picture is clearly not autograph Ramsay, so it should not be attributed to him.
I suspect Jacob is right, we may never get there, but certainly not with this image. If we can't get a better image from Maidstone, let's wrap this one up.