I'm rebuilding a Combat engine and gearbox for a guy. When I did the first phase on my cleaning process on the drive side crankcase, I could not get it to like that they usually do at that point. Today, I did the timing side, which was very dirty, but it came out looking exactly as expected easily like normal. So, I redid the drive side. It came out a little better (I have put in new glass abrasive for the timing side). In the end, it will look fine as very little of the drive side shows when in the frame and he primary is installed.
I think the issue is a slightly different alloy mix at the foundry. They are a matching pair but that matching I'm sure happened long after the sides were cast. For one thing, a quick glance and you would think the drive side is a 1974 850 case it is so porous in places. The timing side it more porous than is usually for 1972 cases but no where near as porous as the drive side. Also, the color is slightly different between the sides. I'm sure once I've finished with the final bead blasting and cleaning the will look the same if you don't look too hard. I've done lots of Norton, Triumph, and BSA cases and while these started out nowhere near as bad as some, the drive side has been a real challenge.