I have had this MS on my wanted list for several years after seeing it listed in Collect Birds on Stamps as featuring Red-billed Oxpecker (Buphagus erythrorhynchus). The Yellow-billed Oxpecker (Buphagus africanus) had already appeared perched on a Rhinoceros in the 1987 Flora and Fauna set.

After a long search, I recently found this MS for sale on the US web site, CoolStamps.com. I was disappointed when the MS arrived to see that the species clearly is Yellow-billed. During my search for images on the web, I came across this one which I believe shows both species of Oxpecker on the rear end of a Zebra.