I have observed the notch less sights on percussion rifles before.
Such sights have yielded very good results with my aging eyes on my own rifles. I can't see the platinum line, so don't bother with it.
They seem to work like an aperture sight, where one's eye naturally centers the bead on the leaf.
A bead somewhat larger than normal is a big help. The size is not a problem blocking the target, as the bullets strike at the top of the bead.
Nice rifle! I read of notchless rear sights with only a platinum line, but never actually saw one. I suppose that could indicate such sights were a short lived fad that were simply found to be less practical and precise than a more conventional rear open sight. Few people approaching middle age would be able to simultaneously focus on the platinum line, the front sight, and the target with any speed or precision.
Mike Rowe's comment above saying the bullets with such an arrangement would strike at the top of the bead seemed a bit problematic, because with my own black powder rifles, the average sight bead can cover a fairly large portion of the target at longer ranges. I had one .50 cal. flintlock that was difficult to get the precision I wanted because the rear notch was narrow, and the bead fairly filled the notch. That made it hard to know I was centered on the heart of a deer at 100 yards, especially in low light conditions, so I modified the rear sight notch. And if my bullet were to hit at the top of the bead at that range, that could mean a shot intended for the heart would result in a very high lung shot, or worse. Or it would mean consciously holding low to make a heart shot, which isn't a great idea either.
I looked at your photos and was surprised to see there wasn't as much difference in height between the 100 yard and the 500 yard blade as I expected, especially for a percussion 12 bore rifle. It seems like there is a lot more difference in height on small bore higher velocity military rifles with ladder type rear sights when you elevate to longer ranges. So it seemed like the sight graduations on your rifle were optimistic at best. I shot my .50 and .54 cal. flintlocks at longer ranges to check the trajectory with hunting loads, and they really start dropping like a rock when you get much past 125-150 yards. I'd be very surprised if your gun shoots to the sights once you find a regulation load, and actually fire it at the longer ranges.
When I was in my 20's, I bought a .69 cal. percussion replica rifle and mold from a friend. I asked him what load he used, and he said 160 grains of FFg with a round ball that weighed around 450 grains, as I recall. That would be a 5.85 dram charge, which in hindsight was probably much too heavy for the gun. The recoil set me back a step or so, but it was a literal blast to shoot. It turned out to have poor accuracy, even when I varied the charge a bit, so I sold it. The trajectory at longer ranges was worse than my smaller caliber flintlocks, and I never tried it past 200 yards. I'd guess a 12 bore round ball trajectory would be even worse.
