If you confirm that there is no sign of rifling in the barrel, I agree that it must have been converted and the markings changed.
A rim recess for 9.3x74R fits very well for 410, in fact there was recently forum on this site concerning the use of 9.3x74R cases to load 410 shot shells. The person trying this reported that the 9.3x74R rims generally the 410-rim recess very well. My cousin is currently working on a double rifle project for a 65 mm long version of a Lancaster type cartridge. He wound up using 9.3x74R cases instead of the 7x65R ones he started with due to losing too many of the 7mm cases expanding the necks. A 16-gram bullet would fit for a 9.3x74R but my hesitation to agree with that is the amount of metal that would have to be removed to go from .365-.366" groove diameter to a .410" bore. I'm not saying it can't be done, just that it would work better with one of the Lancaster express cartridges (an express cartridge usually has a light for caliber bullet and as I recall, research for my cousin's double indicated somewhere around a 240- 250 grain bullet). It would answer the question if there were enough left of the old bore mark to make it out.