Long shot for here but thought I'd give you first dibs !

https://www.newton-equipment.com/aero-filler-cap-range.html
https://www.speedflowshop.co.uk/aero-500-12-bolt-flange-670-512sl-1226-p.asp (ex VAT !)

Brand new, locking, but missing key (available from Newton). Beautiful Newton Aero fill cap. Bought as a secondary cap but just too small for my big hands - ideal for direct tank install. Half price at £70 inc postage





Feel like I should have cleaned the bench and put it on a red velvet cushion!