A rectangle to round plastic adaptor from Rytons.
The rectangle is roughly single air brick size and the round is 100mm (4 inch) diameter.
It is designed to attach with duct tape to the outlet of the periscope plastic duct to allow the air to be transferred below the floor via round 100mm diameter ducting or flexi pipe. It is however more often used with single cavity wall sleeves, periscope ducts and also single air bricks to allow them to feed into FDUCT adapting from rectangle to round. It isn’t a perfect fit with any of these products but is close enough and will need duct tape to create a good seal.