To be exact - a Stadium BM435/1 Brass Standard Floor Ventilator
Unused - just a bit dusty as been on shelf in garage for years - hope someone can use it.
Lots more photos of what it looks line and sizes etc in the full details here:
https://www.gasproducts.co.uk/heating/ventilation/stadium-bm435-1-brass-sta...
Brass Floor Ventilator (Top Amersham HP6)
This offer has been gifted to someone and is no longer available.