You can connect your Mini Monster Reto Gaming Kit to either your BBC Micro Model B/B+/Master or Acorn Electron (with the Plus 1 Module) with this tiny adapter, it fits snugly into the rear analogue port and offers 2 digital joystick connectors.
The adapter is also compatible with most C64, Atari, Amiga, MSX joysticks however it will not work with joysticks or controllers with resistors on the inpit to ground lines. for example it is not compatible with Master System or Mega Drive (Genesis) controllers as they have these and are not compatible with the analogue to digital nature of the adapter.
By Keef from Caterham
Item plugs into the back and you plug your atari joystick into the adaptor. Tested with Arcadians (Galaxian Clone) and works perfectly. Delivered day after ordered and well packaged. I would buy from MonsterJoysticks again :-)
By TomDD from UK
Adapter works very well, only downside is it could do with a case, so I designed one you can 3D print https://www.thingiverse.com/thing:3320298