Well, I am very impressed with Huvius's assessment.
The certification letter from Rigby indicates that it was indeed a complete Best Mauser takedown rifle that was sold to Dickson on August 22, 1901.
So, yes, it is a Rigby made rifle, except for one detail - the takedown system appears to be a Dickson takedown system that used a set screw on the left side of the receiver to secure the barrel.
Which sort of takedown did Rigby use at that time?
Is it possible that Dickson converted it to their own takedown style?
And if so - would there be indications in the stock inletting that a different takedown was installed prior?
My only other explanation of this inconsistency is that it might be possible that Dickson ordered a rifle from Rigby specifically with this sort of takedown.
Looking forward to your comments!