Guy,
I'm probably chiming in too often, but now I'm really curious. Since you are talking about the external appearance of the crankcase, I take it you have not parted the cases. Without parting the cases you could not know if the sludge trap is clear. This leads me to think you may be mistaking the oil sump plate for the sludge trap. (Please pardon me now, if I am vastly underestimating your knowledge.) The sludge trap lives within the crankshaft and runs through both rod journals. It consists of a tube with holes in it that serve to separate, by centrifugal force, particulate that would otherwise stay in the oil and clog up the ways. If the sludge trap gets overloaded over some years, chunks might get released and block the holes leading to the journals, resulting in frozen rod bearings.
Richard L.