I guess someone wanted a black on black Gladstone. Um.
So if you still want to try to remove more polish and you are brave I’ll tell you what worked for an acquaintance: Ronsonol.
Ronsonol is an old brand of lighter fluid. Which sounds scary but no more so than acetone. We used Ronsonol BECAUSE it is an old brand and might have fewer newfangled additives. Ronsonol is very difficult to find in a store nowadays but easy and cheap on
Amazon.
The first thing was saddle soap, then the Ronsonol (don’t squirt it right on the bag, saturate something else (rag, cotton round, t-shirt) and use that to scrub.
It will take off the British Tan dye eventually if you are too violent, but the shoe polish generally really does come off.
If you do have an old-fashioned type camping or hardware store near, Ronsonol comes in a couple sizes of BRIGHT yellow squeeze bottle. You can get a smaller one for five bucks or less if you want to try a small area.