I wanted to share my mod for old, but in great shape, Olympus CH-2.
I was able to acquire CH2-DS Darkfield Central Stop on eBay from a seller in USA for about £13.75 + £14.65 shipping
... I didn't want to create this part without knowing proper dimensions.
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The Mount I was willing to print as it sits between two parts known in size.
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You can grab the file from https://www.thingiverse.com/thing:3400092
I am very happy with the results and I encourage you too to tinker and add a mod to your scope