Aha
Well that book was written in 1997.
After that Dan has produced two more books on Latin American military aircraft (his latest was Conquistadors of the Sky in 2008)
In none of those two books does he mention any Bristol Fighters in Bolivia.
Since there is no lack of photos of Bolivian aircraft post 1925, we are also left with the strange wondering why nobody took any photo of these ten aircraft, which after all must have been the most common aircraft in military service before they were retired. Did everyone in Bolivia aquire cameras only in 1926?
The basis of the information is JANES from 1926. Since Dan is also relying very much on documents from US Military attachés and Embassies, it is strange that none of these sources has revealed anything.
I doubt very much Bolivia had any Bristol Fighters...
Cheers
Stig