This very commonly used device has been very uncommon to find. We now have them! The valve is needed on nearly all twin carb models such as MG
Midget, MGB and of course Cooper S models to pass emissions control tests. A properly operating valve has no adverse effect on performance, but may improve it.
This is the ;ater type with the flat lid as fitted from 1966 on but fits all.
The rubber diaphragm is available as
27H7758.the bracket to hold it was
AEG351 which is nla