This portrait seems to have survived World War I as it popped up at a Hotel Drouot auction in 1920. The listed height went down from 59cm to 49cm and I wonder whether it's been trimmed along the bottom edge. photocollections.courtauld.ac.uk/sec-menu/sea...
The "Search by Image" add-on is also pretty good for reverse image searching - it allows you to right-click on an image and search on Google, Bing, TinEye and Yandex all at once.
If you sign up for an RKD Research membership (€65 per year), you can use the online "visual search" function, which is weirdly hit and miss - sometimes it'll show abstracts in response to portraits - but sometimes useful when all else fails!
RKD suggests Francis Cotes for this one, but without knowing when, how, or by whom this attribution was made I'm not sure how much credence can be given. There's no record on RKD of its existence since 1898. rkd.nl/imageslite/2...
But regarding your original quest, I have no idea where this painting is now!
This painting is listed on the RKD site
rkd.nl/imageslite/9... and rkd.nl/images/214771.
It was sold in Versailles in 1988 and is listed in Bourdon's 2000 catalogue raisonné (pp 498-499) in the section of paintings that can no longer be attributed to him. archive.org/details/seba...
