Pentax 67 helicoid extension

The Helicoid Extension Tube is a handy 6x7 camera accessory which permits free variation of subject magnification, thus making it extremely handy for close-up and copy work with the Pentax 6x7. It fits between the camera body and the lens in use and permits focusing and composition while viewing directly through the viewfinder minimum and maximum magnifications obtained with the Helicoid Extension Tube vary in relation to the lens in use, as do film-to-subject distance, picture area and exposure factors.

More information is in the manual in PDF format here.