Hi Raymond, the Lis Terkol is very interesting Ship to build, i would not recommend it for the Beginner... but if you have built Deans models before then enjoy..
the jig is made out of a solid piece of hardend steal, milled and filed to shape, if you find a punch of the size required then of course that can be used...
because the Jig starts of round which converts gradually to a rectangle, you can push the rings over the jig with your fingers.
the brass tube wall thickness was firstly milled down to about 0.2mm so there is not much brass left over to cause a problem when pushing the rings over the jig
once the ring is rectangle and while it is still on the Jig, using a pair of long nosed pliers to squeeze the corners, so they are a lot sharper..
here is a better/larger photo
hope this helped, and of course don't hesitate to ask more questions, i have over 190 photos of my build