Hi westaussie and welcome.

If it is the actual barrel regulation that you are interested in you could try to get a copy of Alexander Gray's book "The Hammerless Double Rifle" as it gives some details.
You could give Peter Moncrieff Books in Victoria a call on (03)98986684.

Are you wanting to regulate or re-regulate a double or are you just wanting to know how it is done?

I've done a bit of regulating and re-regulating and may be able to help you out regarding the how to (and how not to) of the process.