A well used Sprenger Duo Mullen Mouth Loose Ring Snaffle, 12.5cm (equivalent to 5") with 20mm rubberised mouth.
Mullen mouth rubber type bits are wonderful for horses with busy mouths, the lack of movement creates stability and confidence for the pony. The bottom is ported to create room for the tongue. These types of bits are my go-to for mouth issues on your sensitive souls...a lot of the time with them, less is more!
Unfortunately they don't clean up as nicely as metal bits, this one has discolouration of the rubber but has very little wear / teeth marks. Still very safe and perfectly usable. At this price, if you've ever wanted to try one of these, now is the time.