The scuttlebutt has been that Rigby's (non-Mauser) actions are fabricated by Mayfair Engineering, a specialist English gun trade CAD-CAM fabricator. They used to have a website -
http://www.mayfairengineering.com/ - but it has mysteriously gone off-line.
The forend latch on the Shikari rifle is technically referred to as a Hervey lever, and is a bit of a Rigby design feature. I actually prefer it, as I think that it secures the forend tighter to the barrels than the alternatives - the Deeley and Edge (catch type) or Anson (push button) type. (Although I dare say that after you put enough rounds down the tube, anything will work its way loose.) You can read about the Deeley and Anson types on the Explora blog
here.