On Bell's famous 7x57 :

It is interesting to note that from the records at JRigby and Sons, that he purchased six 7mm Mauser rifles from Rigby in total over the years, but he only purchased his first Rigby made rifle in 7x57 caliber in 1910. This was well after his time in Karamojo, and after the Lado Enclave.

In other words the .275 caliber bolt action(s) that he used from 1902 to 1907 throughout the five years in the Karamojo, during which he shot the bulk of his elephants with a 7x57 rifle, and the rifle he specifialy refers to in "Karamojo Safari" were not a Rigby rifle at all. It was some other make of rifle. (possibly the .275 Rigby cartridge name has made people assume that it was a Rigby rifle also)

He does not specify the make of this rifle other than it is a 7x57 Mauser. His gun licenses from Kenya in that period show .450/400 (Which he writes he only used on his first Karamojo safari).303 Lee Metford or Lee Enfield rifles, and ""single barreled"" Mauser, which means this 7x57 we must assume.

He mentioned that once he could afford it he had his first bespoke rifle made for him, possibly it is the first Rigby in 1907, but also quite possibly it is one of these pther 7mm that he owned earlier.

But the main 7x57 rifle he achieved success with, and the one we would think was his "main" .275 Mauser, was not made by Rigby at all.

Bell was a good customer for J Rigby and sons, but Bell had several rifles that were not Rigby rifles we must remember, such as his Mannlicher Schoenauer carbine in 6.5x54, his other long barreled "beautifully sighted" Mannlicher by Gibbs that he shot much of his plains game with.(This rifle of all probably did the most work for him - from Giraffe to buffalo - all with soft points); his Thomas Bland double in .318 Westley Richards, as well as at least one other .318 on a Mauser action, and what can only have been a Savage 99 in .22 hipower, which he shot 23 buffalo with in West Africa; and of course his Winchester .220 Swift.

The Karamojo .275 could have been from somewhere like Army and Navy or any other maker, but given his close relationship with Fraser in Edinborough, and the fact that he was still buying rifles from the Fraser gunmakers even after Daniel Fraser himself died, (His Mannlicher Schoenauer carbine was from Fraser, around 1908) it could be fair to assume it may have been a Fraser- made Mauser rifle in .275 that was the rifle he used throughout his Karamojo days.

I wonder now if Fraser's sales records still exist, I would love to check through his purchases from them.

I will mention here that the rifle one comes across on the net of WDM Bell, is the famous take down rifle 7x57 that Robert Ruark bought from Rigby (along with his .450) after Bell's death. This rifle he gifted to Mark Selby.
This is the same rifle mentioned above in the 1937 visit to Rigby.

This particular specific 7x57 rifle was bought for the 1923 car journey across Africa to Khartoum with Gerrit Forbes and his brother, a trip that Bell wrote they did no real hunting, being too busy chasing across Africa in the vehicles at breakneck pace. Later, this rifle was obviously being used as a red deer stalking rifle in Scotland, and which he also had fitted a telescopic sight by that time. (Selby had another scope fitted to the rifle on top mounts, but the rifle as he received it had it had a side mount already in place)
This 1923 rifle probably never shot an elephant in Africa in Bell's hands, and the elephant that Gail Selby shot with it in the 1970's may have been the only one. It is more properly regarded as his highland deerstalking rifle.

Somewhere out there someone may have an old English built .275 Mauser that is not a Rigby, but is Bell's most famous African elephant rifle.