This rifle is obviously made to Manfred Orth's take down system. The receiver ring is bored out to take the barrel extension with the locking lug recesses. The barrel is held in place by the foreend, which slides up in the T-shaped rib and is locked in place by the "Deeley latch".

Manfred patented this take down system in 1998, German patent DE198 15 261 A1, and marked rifles of his own make accordingly.

Here is such a rifle in .416 Rigby made by Manfred himself :

In 2000 he made such a takedown rifle in 12.7 x70 Schüler aka .500 Jeffery for Paul Roberts. (This was before Daid Little's = Kynamco's new ".500 Jeffery" was CIP approved) At about the same time, he made another one in .416 Rigby for M. Lüke, the owner of Blaser, Mauser, Rigby and Sauer.
Now, with the patent expired and in the public domain, Rigby obviously used Manfred Orth's design on this rifle. I spoke with Manfred yesterday about this rifle and agreed:
"Imitation is the sincerest kind of flattery."