Sounds like the previous owner was using the AA low recoil rounds on the assumption they would not exceed black powder pressures, but loading the 10.3×60R himself, using black powder.
There are load data sources online for modern bullets; at least
paid load data is available. I am a little wary of that paid source, as his drawings come from QuickDESIGN, so his load data may come from QuickLOAD, which can underestimate peak pressures a bit in some cartridges. If the previous owner didn't blow the gun up with his loads, I would pull down a sample of them to measure the powder charge and check its grain size, and check the bullet dimensions and weight. I would also ask him what brand of black powder he was using. Then, you'll have something you can replicate.