Sounds like Buehler was just quoiting from the proof marks on the barrel. 
In the heyday of the 500 Nitro Express a rifle proofed in Britain would have the proof load marked conspicuously on the barrel as "80 Cordite 570 Max". The 505 Gibbs would have it marked as "90 Cordite 525 Max". The Gibbs would have 6,166 ft pounds of energy. Looks like it was a 505 rifle and if it was a bolt action (the 505 Gibbs was rimless) it could have had a short barrel to keep the OAL short to keep the barrel from hanging up in the bush. You don't want a long barrel hanging up on the vegetation when the wounded lion is coming at you.