The Reversal ring is used to mount your lens with reverse direction. This enhances the lenses quality if you are taking close up shots. Lenese are designed to perform best between infinity and 1 meter distance.
If you go into the macro (close up) range they underperform. This can be eliminated by turning the lens 180 deg. around mounting it with the front lens showing towards the film and rear element pointing outwards.
The first number is your filter thread and the second number with an "E" behind is the screw mount of your camera.
Modern bayonett cameras (e.g. Nikon Bayonet) can be combined with a Nikon bayonet to M42 adapter in which then the reversal ring is screwed.
This adaptor is available from the respective camera manufacturer.
Heliopan does not produce these.
Best combined with macro rings or a bellows.