This is the way I make hinges as small as 5mm square (closed) Materials required are pins (from the wife's needle work box) small brass tube (in which the pin is a good fit) Thin brass sheet (About 1mm thick). Only need a small strip so can file down thicker, silver solder & flux. Silver solder the tube along the edge of the sheet. The tube is longer than the sheet so the solder will not capillary into the ends. Pieces of solder placed in position on the flux before heating, as soon as it runs remove the heat or the brass will melt being so thin. I use fire cement to hold things in position, as soon as the heat is applied it sets, a light tap with a hammer will remove it on completion. The cement is best kept in small quantities in separate plastic bags with a drop of water, to prevent it going off. I did try soft solder but it melted when the knuckles are ground next. 🔨

For this size of hinge, any thing below 8mm a small lathe makes life a lot easier. I have ground knuckles by hand, single disk in a drill and the material clamped in the vice, in much the same manner as this. Take 3 discs mounted in a mandrill the centre disk being worn & much smaller than the outer pair. Clamp the sheet in the tool post with steel packing and the tube exposed half a mm. Line the up so both large disks will be cutting slowly wind the post in until touching the steel packing. Remove wind along until the the inner disk is in line with the slot cut by the outer disk. Wind in shaving just a very little off the knuckle cut on the first pass. I have found this necessary, because the disks are slightly thinner on the outer edge than the worn centre disk. Repeat this process until enough knuckles are cut, pluss a few spare. 🔨

Back on the bench push a pin through the knuckles to hold them straight. The burr from the grinding will hold the pin tight. Mark out the required size hinge ( that is scribed lines not pencil) Drill fixing holes. Only using one nail in each leaf at this size. Cut out with scissors or snips. As can be seen 4.5mm wide I don't think you can buy them that size anywhere. Open & shut. 🔨

Managed to find a photo of the hinges at the construction time of the Duke.