Remember, the reversed lens doesn't have to be
from the current camera system as it is being mounted by its filter thread.

Canon EOS Electronically Coupled Reversing Ring An electronically coupled reversing ring from Novoflex giving auto functions

Canon EOS Reversing Rings for Manual lenses Canon EOS fitting reversing ring to use lenses with manual aperture rings.

Canon FD Reversing Rings Reverse mount Canon FD lenses onto camera body, extension tubes, or bellows

Contax / Yashica Reversing Rings Reverse mount Contax/Yashica lenses onto camera body, extension tubes, or bellows

M42 Reversing Rings Reverse mount M42 screw lenses onto camera body, extension tubes, or bellows

Nikon Reversing Rings Reverse mount Nikon lenses onto camera body, extension tubes, or bellows

Olympus OM Reversing Rings Reverse mount Olympus OM lenses onto camera body, extension tubes, or bellows

T2 mount reversing ring inserts