... first trials. But that is still very thin (5 thou), so I have invented a thicker inner area which I have called the "plinth" rectangle, on which to stand the chair jaws. Hopefully this should look more like the rounded profile of the chair casting after printing. It's thickness can be adjusted by trial and error to get the best effect. All this will be barely noticeable in 4mm scale, but may be visible in the larger scales. The blue objects are your DXF block markers: 2_212217_040000001.png 2_212217_040000002.png They are actually the rail seats. The recess will accept the inner jaw. The projection on the outer side represents the seat area below the key (which supports the rail when it is first dropped into the chairs). The outer jaw butts against the projection. The seat (and hence the DXF block reference) is longer under the wider L1 bridge chairs: 2_212217_190000000.png That's as far as I've got. The next job is to try building the jaws, and fitting the keys. I will post you a DXF ...