I just measured it and came up with 1/4" too. Not sure what that is in metric units though.
So I started putting the bits together and thought I'd share what I did.
For the Hama slide viewer I put a long thin nail through a washer and glued that on the inside pointing out. I then used SteelStik to putty up the outside and painted it black.
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On the Kango side I couldn't get access the inside so instead I glued a large rivet into the hole I'd made for my earlier tubing, and then glued the drilled out model lamp to that. The silver of the rivet rim mimics the washer visible on the original which is nice.
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So here's the final version with the model lamps and correct tubing in place. Please ignore the resin pump widget and the front knob being turned the wrong way, these will get fixed soon.
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Threading a thin piece of wire through the coil from the sphygmomanometer allowed me to get the right curve for the front (compare this to my earlier pic to see why it needed the wire!).
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Much better!
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