Thoughts on this issue? The pedestal mount has no offensive odor but the backskin has a distinct sour odor. I've had it outside on the line and it has not improved. No apparent shedding of the hair, the hide looks gray/pale blue like the others. I know Waterbuck are very oily is it possibly this?

chances are its the oils, rub the skin backside with boraxo & it should clean out the odor

If it's Blue Gray it's a chrome tan and most likey done in africa
What you smell are the natural oils that were not totally degreased and removed by the process. It will not decay but you will always have a musty odor to it.
The oils are actually in the dermis layer of the skin and you will not remove them with a surface cleaner like borax.
Your only chance is an odor blocker like frebreeze