One of my meaner looking Tephros.
Here is a link that might be useful:
Your russellii is a bit different from mine, though according to the description I have, the pads are variable in size... but mine has a green stigma.
I don't know a lot about these. Except that T. russellii got moved to Maihueniopsis ovata :) So I don't know if the green stigma is definitive but if it is then that plant would be a different species such M. glomerata. Both these species, plus M. darwinii, hybridise in the wild, so I guess you pick whichever name you like ;)
CP, your plant appears to have flattened pads, unlike mine and the one drawn in the link below.
Here is a link that might be useful: Desert Tropicals link
Yes Steve that's the first thing I noticed when I looked at your pic, It was only when I checked my shot that I noted the different stigma colour. Thanks S&B, I'll investigate mine a bit further.