I've spent a little more time looking at my equipment and doing some searching (sort of a follow-up to work I did on my SMZ-1500 before the nikon camera system was sort of dropped in my lap).
This is the missing link here, picture from the SMZ-1500 manual:
That "photomicrographic equipment adapter", no part number. Funnily if you zoom way in you can see it says "SMZ-10A" on the side. That has the correct diameter hole to accept the projective eyepiece, and then opens up into a 42mm hole at the top which you can mount the camera on.
That might be a pain to track down. I haven't seen one (not that I've been actively looking though). If you do you might find one used for a decent price, or it may be way overpriced.
Or you can try this instead:
One 38mm to m42 adapter from eBay, m42 spacers and one old bausch and lomb stereo microscope ocular (maybe a pain to find one loose, but b&l parts and parts units are ubiquitous and inexpensive--way easier and cheaper than tracking down the nikon part). The b&l ocular fits snugly inside those spacers and will stay put even when tilted. Put the 2.5x pli projective into the ocular, add more m42 spacers above and adapt your camera with a standard m42 adapter. It may take some experimentation to get the spacing just right, both in positioning the eyepiece and the camera above it, so it might make sense to use m42 helicoids. This is definitely what I will do if I ever sell that Nikon camera setup and put my dslr onto my smz-1500.