Daryl, your brothers' rifle is lovely and no surprise it shoots a treat. I am gradually getting around to re soft soldering my Hollises' underib.
I reckon Sarges' old double has been converted to percussion, I bet it is fitted with Nock patent breeches, see the timber above the lock has been relieved to the flint cock to sit, and the lock plats look like they have been modified. Give the breeches a good soak for a few weeks in 50/50 acetone and auto trans fluid if you are going to have a crack at removing the breeches, but I think a good optic fibre tool would be the go first up, relatively cheap of eBay.
Looks like the maker is J Manton ?