My Japanese Charleville has band springs with studs on all of the bands, including the double fore-end band. The use of a screw to retain that band didn't come about until the M1777. My musket is basically a M1763, but, according to Neumann and identified specimens, the M1763 didn't have a retaining spring on the lower band. This was added on the M1768. As I write this, I see that a description of the '63 by Fuller shows that this model's barrel bands were all retained by studded band springs. Go figure.
The first model to have barrel bands was the M1728 and they were held by friction. The first model to have the springs was the M1754. There should be at least a middle band spring on yours.
Neumann also says that the M1768 was the model that was copied by the U.S. for the M1795 Springfield. I would agree even without measuring the locks, &c. One clue is the bayonet lug. It was moved to the top of the barrel on the '68 model and was on top on all U.S. muskets through the M1816. It was moved back to the bottom on the M1835 and stayed there on the M1842. All reproduction muskets I've seen have the lug on the bottom. The flashpan remained faceted in shape through the M1768. In 1772, this was changed to a rounded pan and in 1777 to a rounded brass pan.
My Charleville has a two piece stock, with the joint being hidden under the lower band. At least on this one, they matched the wood well, though a close look will reveal some mis-matched grain. You have to remove the band to really tell the difference
There were a lot of variations during this period, not to mention specimens turning up that had been altered in some way while being repaired during and after the war. I'm sure that the guys who set up the patterns for reproduction muskets have missed some things or have been confused and these companies are in some cases actually turning out models that never really existed back in the day.
As for what the Japanese musket is based on, I'd go with the M1763. First, the forward band on the M1766 is shaped differently. The rear of it slopes back, much like the band on the M1842 US musket. The lip that appeared on the M1763 disappeared on the M1766. My Charleville has one. It also has a fairly thick buttstock which was slimmed down on the '66. There is only 1/8" difference in the length of the lock and 1/4" difference in the triggerguard, so I don't figure they paid much attention to that detail, and as long as "thread counters" don't start measuring locks and other hardware, I wouldn't worry about it.
If someone really wanted to, he could modify a few things to make his musket to be as close as possible to the real thing. I've added a few marks and stamps where needed just for the heck of it, but this ain't critical. Main thing is, they're a hoot to shoot!
Just saw your last post. Mine had Japan stamped on top of the barrel between the breech and the lower band and a 4 digit s/n on the left side at the breech. Of course that's all gone now.