Hi Brian,
Here is my modification of a civilian guard to mount a swivel for an officer's fusil. Keep in mind, part of my design was meant to fix the casting, which broke in front of the bow but the basic idea is an option for you.
simbas uncleLet me identify the pieces for you in the first photo. The lug with threaded post in front of the guard bow is the boss through which the swivel screw will go. It goes through a hole in the front extension of the guard and the lug, shown below and to the right of the bow, is drilled and tapped to receive the threaded post like a nut. It also doubles as the forward lug for the guard anchoring it to the stock. The sheet brass plate is a spline that connects the forward extension to the bow and is soldered to the forward extension and the forward curl of the bow. The assembly withe the swivel boss is soldered together and then a brass rivet is drilled through the swivel boss above the swivel screw hole and into the bow and peened in place. A second brass rivet is drilled through the spline plate and the forward curl of the bow and peened. The assembly seems to be very strong and looks like several original guns.
dave