Dave,
Ive got the opposite situation. At the moment I'm running stainless steel rectangulars and I want to change to rounds ( aesthetic, nothing else)
As you know, I use Mike Duncan for a lot of my stuff and John W at HoE is next door.
I've seen John's round mirrors but what strikes me is that every set I've seen on a car, the centre retaining screw through the back plate is rusty. My conclusion- that bit of the mirror ain't stainless.
Ben D has come up with a nice 'slant' on this. We're planning to use Suzuki m/bike mirrors. These are round, v. good quality stainless and don't require the stalk mounting to be replaced.
I'll keep you up to speed as we progress
regards,
G