This is a print-in-place telescoping Microscope adapter for Nikon crop DSLRs. It telescopes from 2.75X to 4x magnification

To see how to assemble it visit: https://youtu.be/MEB0Ze5Yfpc

It uses this $20 microscope objective: https://www.amazon.com/gp/product/B01G4CCGRC/

For a review see: https://www.closeuphotography.com/seventeen-dollar-plan-4x-objective/2017/4/3/17-plan-4x-objective

I have another one of these that doesn't telescope and is for full frame Nikon F-mount cameras at: https://www.thingiverse.com/thing:4259543

for other cameras like Canon, Fuji, m42 (or what you can adapt from m42) see thenickdude's adapter https://www.thingiverse.com/thing:5130912

If you just want to buy one it's for sale at: https://www.etsy.com/listing/1433104167