Ethan-

You're totally welcome, and I'll push the recommendation to get active here on sorting yourself out with that rifle and sorting the rifle out for work.
Load for it, and get your loading bench and technique tuned for this neat new cruise you're embarking on.

I'm guessing that the screw on that rear trigger's had it's head cropped off at the request of a previous owner, finished and blacked to it's current appearance. It doesn't look to simply have had it's head broken off. You'll see them capstan-headed too, instead of slotted. It's easier to get in there with a capstan pin than it is to get a screwdriver in there without buggering the screw head or the trigger guard (or both)

Remember, I did note that there are solids that are designed specifically for double rifles -- I've never run them myself, but there are some here who have.
You'll need to slug your bores (from both ends) with pure soft oversize roundball and a leather or wood mallet and dowel to get your exact bore/groove measurements before you consider going that route.

G S Custom Bullets is one of the bullet manufacturers I speak of. Start a thread asking about handloading for your new 9.3x74R here and ask about them. I don't know enough about them to say 'go'

On working at the shooting bench, all the shooting I'd done with my 8x60r and with the 9.3mm rifle (and others like it) I noted above was done standing, with a support side toe and elbow against one of the support posts of the shooting bench.
I do fairly well shooting a rifle offhand, and that's worked out well with those rifles.
A common approach for supported, seated shooting at a bench is to sit straight up assuming upper body posture that resembles your standing posture while in a hunting field position, and for rest, have a sandbag that's supported at the right height to give the back or heel of your support hand a rest, that support hand around the barrels (often ahead of the forend for most guys) at the firing of the rifle.
That tends to effect the rifle the least and it shows results on paper very similar if not the same to your results in field positions.
That's the method I've taken to get some of my other rifles running with 'regulation' loads.

The load that your rifle shoots most accurately, near or at the 'factory ammunition velocity', with both barrel's groups flying with parallel trajectories (essentially superimposed groups-it'll look like a one-barrel gun group) is your 'regulation load'
The process by which the gunbuilder took to get the gun to shoot such a load is known as 'regulation' or 'regulating the rifle'

Do all of your shooting over a chronograph.
Know what that factory ammunition is doing out of the muzzle of your rifle. If it shoots well, you're there already and have a base line to work with in the future for your handloaded ammunition.
I have learned to take a rifle that runs cheap ammunition at a consistent velocity to the range or ranch with me whenever I shoot the double rifles. This gives me a base to start with for the chronograph - and lets me know if it's even going to work that day, without using up valuable double rifle ammunition or double rifle barrel time.

Have fun with it too!
It looks like a really sweet piece, I bet it handles very nicely in the hands.

--Tinker