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 Reversing Rings These enable lenses with manual aperture rings to be mounted onto EOS bodies to enable macro photography with a standard lens.

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

Sony Alpha Reversing Rings Reverse mount Minolta AF/Sony Alpha lenses onto camera body

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

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

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

T2 mount reversing ring inserts

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