Hi Scot,
They were made by filing a step into the bolster by the frizzen screw as you suggested. I don't think the bolster on the L&R lock is thick enough to allow you to do that and still have enough threads for the frizzen screw. Not all Manton pistols had recessed breeches so a small chambered breech without a recess would be fine. Unfortunately, the L&R lock makes an awkward looking pistol if the pan is level with the barrel. The nose tilts down because of the curve on the bottom an you end up with a distinctive "knuckle" formed by the lock molding around the front of the lock. The lock is also too long to get the right architecture. You might be better off using Rice's "Nock" lock. If you are trying to make dueling pistols keep in mind they were pretty plain guns but with the highest quality workmanship, finish, and technical function. The stepped breech moves the pan closer to the powder in the powder chamber, which also has a liner of gold or platinum. It also allows the stock to have a cross section shaped like a vertical oval through the handle, which fits the hand nicely.
dave