Very clever!
I think you've just made me realise why my Russells are not to be found.
Bucks is on the 1911 Beta test but Oxfordshire is marked as 'coming soon'. Following your links, I rediscovered (I should have remembered this from birth certs etc , but , in my own defence, it's been a while!) that, although Medmenham is actually in Bucks, it comes under the Henley Reg District - and, of course, Henley is in Oxon.
So that's that mystery solved.
Thanks very much for helping me sort that out!
...and relax!!!

Jill