I too just picked up a Field's patent rifle but in 577/500.
I agree that they are a very interesting action and just about as simple as one can get. I love how by simply backing out the keeper screw on the left the entire block can be removed from the top and the firing pin serviced if need be.
The one I have I believe was made entirely by Field and has a very early number - earlier than any listed in Kirton's book. Can't say as to what the number is offhand since it is out getting some stock repair and a general sprucing up.
Will post on it when I am able to.