Yep, loaner or renter, but not his own. I am looking too. It might be in the collection of articles compiled by Michael McIntosh and published as
Robert Ruark's Africa.
In the meantime I found the part where he talks about the rifle's sears being "tricky" in
Use Enough Gun p. 242 and 243. The writing leads us to believe that he used the same rifle (Westley Richards .470) in subsequent safaris after his first safari, because the following hunt takes place in the first hours of the first morning of the first day of this safari, and he obviously already knows this specific rifle well...
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We know from later writing that Ruark bought a Jeffery in .450/400 for his godson, Mark Selby. The rifle had been previously owned by W.D.M. “Karamojo” Bell. Ruark used this rifle on his last safari with Harry Selby, the horse and camel back "Kwaheri Safari", because it was lighter and more adapted to ridding in a saddle scabbard...
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https://www.americanrifleman.org/articles/2018/2/26/the-kwaheri-safari-part-i