The old .43 Mauser round would have been called (if it had been American), the .44-77-385. Original loading was a .443" slug of 386 grains weight. Bore diameter suposedly was .433, but slug was nice and oversize. Charge was 77 grains. It is listed in several old manuals with this loading. Except for shape of case, you can use exactly the same loading for the .43 Spanish.
Dominion cases were engineered that they would work through the '71 Mauser, which took the original loading, through the later loading with the flat-tip slug in the 71/84 AND through the Spanish Remingtons. Very nice thing to have. Commercial Dominion stuff like I used to get at the hardware store when I was in high-school ($3.85 a box) was loaded with a light charge of smokeless powder. If you're loading with smokeless, you can use SR-4759 powder. It works fine in lots of old blackpowder rounds; just use it at THIRTY-EIGHT PERCENT OF THE BLACKPOWDER CHARGE and you should keep all your fingers, given that the gun is safe to begin with. Your charge for the Mauser will be JUST UNDER 30 grains; if your scale goes over that, you've done something wrong.
Or you could just load blackpowder, boil the crud outta your cases just as soon as you get them home. This smokeless powder's just a fad, anyway!