"Usual Marlin markings"??? Usual for a Ballard was nothing but a serial number under the forearm on JM Marlin made Ballards. On later Marlin Firearms Co. made Ballards they added serial number, and caliber.
But I've never seen a Marlin Ballard with anything more than serial number and caliber. Does yours have more than a serial number?
If the bore is bad, I'd not waste time on reworking the extractor myself. I'd have it bored or relined, and then make the extractor match the new bore and chamber. Since it's a .40-85 now, I'd have it relined to the same, but have the chamber cut to use 9.3x74R brass formed out to .40-85. This way the extractor, and the chamber would fit perfect, and it would remain the original marked caliber.
I'm contemplating fitting a spare extractor to mine to fit the 9.3 case rim better. Then put the original extractor in the safe and mark what it's for.