Having received no answers to my question, I took it upon myself to locate an owner of an actual Jeffery rifle and managed to find one in (of course) Oz. Here is his response:

Bill

The underlever snap it is said to be (much) stronger than the toplever but I always thought that was only true of the Jones rotary U/L so I am not sure. Sounds a bit like marketing by Jeffery to me!! A combination of physics and geometry may produce the answer why I suggest as I am not sure on the snap U/L.

It is cocked by the barrels opening as per conventional T/L.

Regards

Fred Roberts

TR Group Pty Limited

Tel: +61 2 8289 5500

Fax: +61 2 8289 5501

Mobile: 0414 995 214

Email: froberts@trgroup.com.au

His web site (www.bestgunsaustralia.com.au/doubles.htm) has a lot of really nice guns, which are unfortunately well outside my price range.