These are reticle holders, made by Martin Microscope, needed for Wild and Leica M series stereomicroscope eyepieces. The bottom of the Wild / Leica eyepiece unscrews, the reticle sits on the part that comes out, and one of these holders / caps is required to hold the reticle (ocular micrometer) securely in place. Then, the whole thing threads back into the bottom of the eyepiece. Older Wild and Leica M series scopes use the 23.5mm holder, but newer Leica M series eyepieces use the 25mm holder.
A standard 10mm / 100 divisions reticle may now be ordered with the holder if desired. We sell numerous types of reticles not listed on this website, so please contact us for other options.