JMC,

I looked at (and regret that I did not purchase)a used 20 ga CSMC CE at the 09 Vintagers shoot in MD. This gun had the flat back action & I was told the customer who ordered the gun wanted it exactly like the 20 ga CE on page 134 of McIntosh's book except for having 2 triggers. I was looking for a 16 ga & ordered a 16 ga from CSMC at the shoot. They would not build me one with a flat back action though that is what I wanted.
My point is that they have built at least one gun with a flat back action in the past & with the current cnc machineing of the actions I don't see why it would be difficult to do so.
That particular CE 20 ga was one of the nicest CSMC Fox guns I've seen & I don't remember the serial # or when it was built.