Ian you are spot on, Burlen has expanded their machine shop due to supply problem from out sourcing. I had a batch of their new aluminium MK1 30 mil concentrics and weighed them. Old zinc style 990grams and new type 645 grams. They had been supplying the aluminium ones for BSA rocket 3's for some time. Guys complain to me in Australia as they have to wait up to 6 months for a pre monobloc such as a 1950 AJS 500 if not on the shelf in UK. People do not realise that after Grosvenor Works closed down you could not buy the carburetters until Burlen took it up many years later. They also do not realise that the 276 comes in 3 different internal sizes being 15/16, 1 inch and 1 and 1/16 and that the bowls can be top or bottom feed and on various angles left or right. If Burlen closed tomorrow we would be in big trouble. They also come with a 2 year warranty which out of the hundreds of carburetters that I have sold I have only had one returned to me which had a bit of swarf in it so was soon fixed. I visited Ireland and UK about six years ago and did a tour of the factory and watched a guy drill out main jets and then use a go and no go gauge to check them. What a tedious job but he was proud of the fact that his finished product got fitted to motorcycles around the world.