Alan

A friend of mine has a .350 Rigby Magnum - in an almost unfired mint condition Rigby. He purchased it on the condition by the seller that he could not fire it.

I think the flanged version of the rimless .350 Rigby Magnum is what you are referring to, and is also known as the .350 No. 2 Rigby. The only difference in the case between the .350 Rigby Magnum and the .350 No. 2 Rigby is the first is rimless and the second is flanged.

Would make a nice medium double rifle calibre especially if the double is handy and light, don't you think?