These are top quality and made exclusively by Lambretta upgrades to suit the LTH, Atomic etc aftermarket reed block conversion manifolds.  this sits between the manifold and barrel.
 
We have also noticed that the ones provided were not big enough for some 'overtuned' barrels so we made them bigger, we also doubled the thicknes to 1mm.  The internal hole is as standard so will need opening out to match your manifold.  The stud spacings are for the bigger 200cc+ kits, but you can elongate them to suit the smaller ones, as you have to with the manifolds.